diff --git a/internal/checker/flow.go b/internal/checker/flow.go
index 1952dd2ba9..8df30cad40 100644
--- a/internal/checker/flow.go
+++ b/internal/checker/flow.go
@@ -2405,7 +2405,7 @@ func (c *Checker) typeMaybeAssignableTo(source *Type, target *Type) bool {
 func (c *Checker) getTypePredicateArgument(predicate *TypePredicate, callExpression *ast.Node) *ast.Node {
 	if predicate.kind == TypePredicateKindIdentifier || predicate.kind == TypePredicateKindAssertsIdentifier {
 		arguments := callExpression.Arguments()
-		if int(predicate.parameterIndex) < len(arguments) {
+		if predicate.parameterIndex >= 0 && int(predicate.parameterIndex) < len(arguments) {
 			return arguments[predicate.parameterIndex]
 		}
 	} else {
@@ -2496,7 +2496,7 @@ func (c *Checker) isReachableFlowNodeWorker(f *FlowState, flow *ast.FlowNode, no
 		case flags&ast.FlowFlagsCall != 0:
 			if signature := c.getEffectsSignature(flow.Node); signature != nil {
 				if predicate := c.getTypePredicateOfSignature(signature); predicate != nil && predicate.kind == TypePredicateKindAssertsIdentifier && predicate.t == nil {
-					if arguments := flow.Node.Arguments(); int(predicate.parameterIndex) < len(arguments) && c.isFalseExpression(arguments[predicate.parameterIndex]) {
+					if arguments := flow.Node.Arguments(); predicate.parameterIndex >= 0 && int(predicate.parameterIndex) < len(arguments) && c.isFalseExpression(arguments[predicate.parameterIndex]) {
 						return false
 					}
 				}
diff --git a/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.errors.txt b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.errors.txt
new file mode 100644
index 0000000000..39b13ba38a
--- /dev/null
+++ b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.errors.txt
@@ -0,0 +1,23 @@
+assertsPredicateParameterMismatch.ts(7,12): error TS1225: Cannot find parameter 'condition'.
+
+
+==== assertsPredicateParameterMismatch.ts (1 errors) ====
+    // This test verifies that the checker doesn't panic when an assertion predicate
+    // references a parameter name that doesn't match any actual function parameter.
+    // This specifically tests the code path in isReachableFlowNodeWorker.
+    
+    function assertCondition(
+      _condition: boolean
+    ): asserts condition {  // "condition" doesn't match parameter "_condition"
+               ~~~~~~~~~
+!!! error TS1225: Cannot find parameter 'condition'.
+      if (!_condition) {
+        throw new Error('Condition failed');
+      }
+    }
+    
+    function test(): void {
+      assertCondition(false);
+      console.log("unreachable");
+    }
+    
\ No newline at end of file
diff --git a/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.symbols b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.symbols
new file mode 100644
index 0000000000..bbeb60ccf3
--- /dev/null
+++ b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.symbols
@@ -0,0 +1,34 @@
+//// [tests/cases/compiler/assertsPredicateParameterMismatch.ts] ////
+
+=== assertsPredicateParameterMismatch.ts ===
+// This test verifies that the checker doesn't panic when an assertion predicate
+// references a parameter name that doesn't match any actual function parameter.
+// This specifically tests the code path in isReachableFlowNodeWorker.
+
+function assertCondition(
+>assertCondition : Symbol(assertCondition, Decl(assertsPredicateParameterMismatch.ts, 0, 0))
+
+  _condition: boolean
+>_condition : Symbol(_condition, Decl(assertsPredicateParameterMismatch.ts, 4, 25))
+
+): asserts condition {  // "condition" doesn't match parameter "_condition"
+  if (!_condition) {
+>_condition : Symbol(_condition, Decl(assertsPredicateParameterMismatch.ts, 4, 25))
+
+    throw new Error('Condition failed');
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+  }
+}
+
+function test(): void {
+>test : Symbol(test, Decl(assertsPredicateParameterMismatch.ts, 10, 1))
+
+  assertCondition(false);
+>assertCondition : Symbol(assertCondition, Decl(assertsPredicateParameterMismatch.ts, 0, 0))
+
+  console.log("unreachable");
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+}
+
diff --git a/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.types b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.types
new file mode 100644
index 0000000000..1f05e06e53
--- /dev/null
+++ b/testdata/baselines/reference/compiler/assertsPredicateParameterMismatch.types
@@ -0,0 +1,41 @@
+//// [tests/cases/compiler/assertsPredicateParameterMismatch.ts] ////
+
+=== assertsPredicateParameterMismatch.ts ===
+// This test verifies that the checker doesn't panic when an assertion predicate
+// references a parameter name that doesn't match any actual function parameter.
+// This specifically tests the code path in isReachableFlowNodeWorker.
+
+function assertCondition(
+>assertCondition : (_condition: boolean) => asserts condition
+
+  _condition: boolean
+>_condition : boolean
+
+): asserts condition {  // "condition" doesn't match parameter "_condition"
+  if (!_condition) {
+>!_condition : boolean
+>_condition : boolean
+
+    throw new Error('Condition failed');
+>new Error('Condition failed') : Error
+>Error : ErrorConstructor
+>'Condition failed' : "Condition failed"
+  }
+}
+
+function test(): void {
+>test : () => void
+
+  assertCondition(false);
+>assertCondition(false) : void
+>assertCondition : (_condition: boolean) => asserts condition
+>false : false
+
+  console.log("unreachable");
+>console.log("unreachable") : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>"unreachable" : "unreachable"
+}
+
diff --git a/testdata/baselines/reference/compiler/typePredicateParameterMismatch.errors.txt b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.errors.txt
new file mode 100644
index 0000000000..6c3037be78
--- /dev/null
+++ b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.errors.txt
@@ -0,0 +1,25 @@
+typePredicateParameterMismatch.ts(10,4): error TS1225: Cannot find parameter 'value'.
+
+
+==== typePredicateParameterMismatch.ts (1 errors) ====
+    // This test verifies that the checker doesn't panic when a type predicate
+    // references a parameter name that doesn't match any actual function parameter.
+    
+    type TypeA = { kind: 'a' };
+    type TypeB = { kind: 'b' };
+    type UnionType = TypeA | TypeB;
+    
+    function isTypeA(
+      _value: UnionType
+    ): value is TypeA {  // "value" doesn't match parameter "_value"
+       ~~~~~
+!!! error TS1225: Cannot find parameter 'value'.
+      return true;
+    }
+    
+    function test(input: UnionType): void {
+      if (isTypeA(input)) {
+        console.log(input.kind);
+      }
+    }
+    
\ No newline at end of file
diff --git a/testdata/baselines/reference/compiler/typePredicateParameterMismatch.symbols b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.symbols
new file mode 100644
index 0000000000..6efd825513
--- /dev/null
+++ b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.symbols
@@ -0,0 +1,51 @@
+//// [tests/cases/compiler/typePredicateParameterMismatch.ts] ////
+
+=== typePredicateParameterMismatch.ts ===
+// This test verifies that the checker doesn't panic when a type predicate
+// references a parameter name that doesn't match any actual function parameter.
+
+type TypeA = { kind: 'a' };
+>TypeA : Symbol(TypeA, Decl(typePredicateParameterMismatch.ts, 0, 0))
+>kind : Symbol(kind, Decl(typePredicateParameterMismatch.ts, 3, 14))
+
+type TypeB = { kind: 'b' };
+>TypeB : Symbol(TypeB, Decl(typePredicateParameterMismatch.ts, 3, 27))
+>kind : Symbol(kind, Decl(typePredicateParameterMismatch.ts, 4, 14))
+
+type UnionType = TypeA | TypeB;
+>UnionType : Symbol(UnionType, Decl(typePredicateParameterMismatch.ts, 4, 27))
+>TypeA : Symbol(TypeA, Decl(typePredicateParameterMismatch.ts, 0, 0))
+>TypeB : Symbol(TypeB, Decl(typePredicateParameterMismatch.ts, 3, 27))
+
+function isTypeA(
+>isTypeA : Symbol(isTypeA, Decl(typePredicateParameterMismatch.ts, 5, 31))
+
+  _value: UnionType
+>_value : Symbol(_value, Decl(typePredicateParameterMismatch.ts, 7, 17))
+>UnionType : Symbol(UnionType, Decl(typePredicateParameterMismatch.ts, 4, 27))
+
+): value is TypeA {  // "value" doesn't match parameter "_value"
+>TypeA : Symbol(TypeA, Decl(typePredicateParameterMismatch.ts, 0, 0))
+
+  return true;
+}
+
+function test(input: UnionType): void {
+>test : Symbol(test, Decl(typePredicateParameterMismatch.ts, 11, 1))
+>input : Symbol(input, Decl(typePredicateParameterMismatch.ts, 13, 14))
+>UnionType : Symbol(UnionType, Decl(typePredicateParameterMismatch.ts, 4, 27))
+
+  if (isTypeA(input)) {
+>isTypeA : Symbol(isTypeA, Decl(typePredicateParameterMismatch.ts, 5, 31))
+>input : Symbol(input, Decl(typePredicateParameterMismatch.ts, 13, 14))
+
+    console.log(input.kind);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>input.kind : Symbol(kind, Decl(typePredicateParameterMismatch.ts, 3, 14), Decl(typePredicateParameterMismatch.ts, 4, 14))
+>input : Symbol(input, Decl(typePredicateParameterMismatch.ts, 13, 14))
+>kind : Symbol(kind, Decl(typePredicateParameterMismatch.ts, 3, 14), Decl(typePredicateParameterMismatch.ts, 4, 14))
+  }
+}
+
diff --git a/testdata/baselines/reference/compiler/typePredicateParameterMismatch.types b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.types
new file mode 100644
index 0000000000..92c77ed7e9
--- /dev/null
+++ b/testdata/baselines/reference/compiler/typePredicateParameterMismatch.types
@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/typePredicateParameterMismatch.ts] ////
+
+=== typePredicateParameterMismatch.ts ===
+// This test verifies that the checker doesn't panic when a type predicate
+// references a parameter name that doesn't match any actual function parameter.
+
+type TypeA = { kind: 'a' };
+>TypeA : TypeA
+>kind : "a"
+
+type TypeB = { kind: 'b' };
+>TypeB : TypeB
+>kind : "b"
+
+type UnionType = TypeA | TypeB;
+>UnionType : UnionType
+
+function isTypeA(
+>isTypeA : (_value: UnionType) => value is TypeA
+
+  _value: UnionType
+>_value : UnionType
+
+): value is TypeA {  // "value" doesn't match parameter "_value"
+  return true;
+>true : true
+}
+
+function test(input: UnionType): void {
+>test : (input: UnionType) => void
+>input : UnionType
+
+  if (isTypeA(input)) {
+>isTypeA(input) : boolean
+>isTypeA : (_value: UnionType) => value is TypeA
+>input : UnionType
+
+    console.log(input.kind);
+>console.log(input.kind) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>input.kind : "a" | "b"
+>input : UnionType
+>kind : "a" | "b"
+  }
+}
+
diff --git a/testdata/tests/cases/compiler/assertsPredicateParameterMismatch.ts b/testdata/tests/cases/compiler/assertsPredicateParameterMismatch.ts
new file mode 100644
index 0000000000..9491593b96
--- /dev/null
+++ b/testdata/tests/cases/compiler/assertsPredicateParameterMismatch.ts
@@ -0,0 +1,19 @@
+// @strict: true
+// @noemit: true
+
+// This test verifies that the checker doesn't panic when an assertion predicate
+// references a parameter name that doesn't match any actual function parameter.
+// This specifically tests the code path in isReachableFlowNodeWorker.
+
+function assertCondition(
+  _condition: boolean
+): asserts condition {  // "condition" doesn't match parameter "_condition"
+  if (!_condition) {
+    throw new Error('Condition failed');
+  }
+}
+
+function test(): void {
+  assertCondition(false);
+  console.log("unreachable");
+}
diff --git a/testdata/tests/cases/compiler/typePredicateParameterMismatch.ts b/testdata/tests/cases/compiler/typePredicateParameterMismatch.ts
new file mode 100644
index 0000000000..c4729d1995
--- /dev/null
+++ b/testdata/tests/cases/compiler/typePredicateParameterMismatch.ts
@@ -0,0 +1,21 @@
+// @strict: true
+// @noemit: true
+
+// This test verifies that the checker doesn't panic when a type predicate
+// references a parameter name that doesn't match any actual function parameter.
+
+type TypeA = { kind: 'a' };
+type TypeB = { kind: 'b' };
+type UnionType = TypeA | TypeB;
+
+function isTypeA(
+  _value: UnionType
+): value is TypeA {  // "value" doesn't match parameter "_value"
+  return true;
+}
+
+function test(input: UnionType): void {
+  if (isTypeA(input)) {
+    console.log(input.kind);
+  }
+}