diff --git a/src/com/google/javascript/jscomp/GlobalTypeInfo.java b/src/com/google/javascript/jscomp/GlobalTypeInfo.java
index dd2ec86c4f2..d2eaec23c75 100644
--- a/src/com/google/javascript/jscomp/GlobalTypeInfo.java
+++ b/src/com/google/javascript/jscomp/GlobalTypeInfo.java
@@ -565,6 +565,26 @@ private void checkAndFinalizeNominalType(RawNominalType rawType) {
       rawType.addProtoProperty(pname, null, resultType, false);
     }
 
+    // Warn when inheriting from incompatible IObject types
+    if (rawType.inheritsFromIObject()) {
+      JSType wrapped = rawType.getInstanceAsJSType();
+      if (wrapped.getIndexType() == null) {
+        warnings.add(JSError.make(
+            rawType.getDefSite(),
+            SUPER_INTERFACES_HAVE_INCOMPATIBLE_PROPERTIES,
+            rawType.getName(),
+            "IObject<K,V>#index",
+            "the keys K have types that can't be joined."));
+      } else if (wrapped.getIndexedType() == null) {
+        warnings.add(JSError.make(
+            rawType.getDefSite(),
+            SUPER_INTERFACES_HAVE_INCOMPATIBLE_PROPERTIES,
+            rawType.getName(),
+            "IObject<K,V>#index",
+            "the values V should have a common subtype."));
+      }
+    }
+
     // Warn for a prop declared with @override that isn't overriding anything.
     for (String pname : nonInheritedPropNames) {
       PropertyDef propDef = propertyDefs.get(rawType, pname);
diff --git a/src/com/google/javascript/jscomp/NewTypeInference.java b/src/com/google/javascript/jscomp/NewTypeInference.java
index 3ed1c4d6af8..5d7eb2c5cdd 100644
--- a/src/com/google/javascript/jscomp/NewTypeInference.java
+++ b/src/com/google/javascript/jscomp/NewTypeInference.java
@@ -2024,7 +2024,7 @@ private EnvTypePair analyzeGetElemFwd(
         pair = analyzeExprFwd(
             index, pair.env, indexType.isBottom() ? JSType.UNKNOWN : indexType);
         mayWarnAboutBadIObjectIndex(index, recvType, pair.type, indexType);
-        pair.type = recvType.getIndexedType();
+        pair.type = getIndexedTypeOrUnknown(recvType);
         return pair;
       } else if (index.isString()) {
         return analyzePropAccessFwd(receiver, index.getString(), inEnv,
@@ -2831,6 +2831,13 @@ private boolean mayWarnAboutBadIObjectIndex(Node n, JSType iobjectType,
     return false;
   }
 
+  // Don't always wrap getIndexedType calls with this function. Only do it when
+  // you want to pass around the result type and it has to be non-null.
+  private JSType getIndexedTypeOrUnknown(JSType t) {
+    JSType tmp = t.getIndexedType();
+    return tmp == null ? JSType.UNKNOWN : tmp;
+  }
+
   private EnvTypePair analyzePropAccessFwd(Node receiver, String pname,
       TypeEnv inEnv, JSType requiredType, JSType specializedType) {
     QualifiedName propQname = new QualifiedName(pname);
@@ -3421,7 +3428,7 @@ private EnvTypePair analyzeGetElemBwd(
     JSType indexType = recvType.getIndexType();
     if (indexType != null) {
       pair = analyzeExprBwd(index, pair.env, indexType);
-      pair.type = recvType.getIndexedType();
+      pair.type = getIndexedTypeOrUnknown(recvType);
       return pair;
     }
     if (index.isString()) {
@@ -3822,7 +3829,7 @@ private LValueResultFwd analyzeIObjectElmLvalFwd(
     if (mayWarnAboutBadIObjectIndex(prop, recvLvalue.type, pair.type, indexType)) {
       return new LValueResultFwd(pair.env, JSType.UNKNOWN, null, null);
     }
-    JSType inferred = recvLvalue.type.getIndexedType();
+    JSType inferred = getIndexedTypeOrUnknown(recvLvalue.type);
     JSType declared = null;
     if (recvLvalue.declType != null) {
       JSType receiverAdjustedDeclType =
diff --git a/src/com/google/javascript/jscomp/newtypes/NominalType.java b/src/com/google/javascript/jscomp/newtypes/NominalType.java
index 902588b28eb..a5883ed4041 100644
--- a/src/com/google/javascript/jscomp/newtypes/NominalType.java
+++ b/src/com/google/javascript/jscomp/newtypes/NominalType.java
@@ -107,6 +107,10 @@ JSType getIndexedType() {
     return foundIObject ? result : null;
   }
 
+  boolean inheritsFromIObjectReflexive() {
+    return this.rawType.inheritsFromIObjectReflexive();
+  }
+
   boolean isClassy() {
     return !isFunction() && !isBuiltinObject();
   }
@@ -241,9 +245,9 @@ public JSType getPrototype() {
   }
 
   public ImmutableSet<NominalType> getInstantiatedInterfaces() {
-    Preconditions.checkState(this.rawType.isFinalized());
     ImmutableSet.Builder<NominalType> result = ImmutableSet.builder();
     for (NominalType interf : this.rawType.getInterfaces()) {
+      Preconditions.checkState(interf.rawType.isFinalized());
       result.add(interf.instantiateGenerics(typeMap));
     }
     return result.build();
diff --git a/src/com/google/javascript/jscomp/newtypes/RawNominalType.java b/src/com/google/javascript/jscomp/newtypes/RawNominalType.java
index 57c4d640b06..177c5936e86 100644
--- a/src/com/google/javascript/jscomp/newtypes/RawNominalType.java
+++ b/src/com/google/javascript/jscomp/newtypes/RawNominalType.java
@@ -243,14 +243,13 @@ boolean hasAncestorInterface(RawNominalType ancestor) {
     }
   }
 
-  private boolean inheritsFromIObject() {
-    Preconditions.checkState(!this.isFinalized);
+  boolean inheritsFromIObjectReflexive() {
     if (isBuiltinWithName("IObject")) {
       return true;
     }
     if (this.interfaces != null) {
       for (NominalType interf : this.interfaces) {
-        if (interf.getRawNominalType().inheritsFromIObject()) {
+        if (interf.inheritsFromIObjectReflexive()) {
           return true;
         }
       }
@@ -258,6 +257,10 @@ private boolean inheritsFromIObject() {
     return false;
   }
 
+  public boolean inheritsFromIObject() {
+    return !isBuiltinWithName("IObject") && inheritsFromIObjectReflexive();
+  }
+
   /** @return Whether the interface can be added without creating a cycle. */
   public boolean addInterfaces(ImmutableSet<NominalType> interfaces) {
     Preconditions.checkState(!this.isFinalized);
@@ -274,7 +277,7 @@ public boolean addInterfaces(ImmutableSet<NominalType> interfaces) {
     // TODO(dimvar): When a class extends a class that inherits from IObject,
     // it should be unrestricted.
     for (NominalType interf : interfaces) {
-      if (interf.getRawNominalType().inheritsFromIObject()) {
+      if (interf.getRawNominalType().inheritsFromIObjectReflexive()) {
         this.objectKind = ObjectKind.UNRESTRICTED;
       }
     }
diff --git a/test/com/google/javascript/jscomp/NewTypeInferenceES5OrLowerTest.java b/test/com/google/javascript/jscomp/NewTypeInferenceES5OrLowerTest.java
index 8a475c7654b..071f92cd9cc 100644
--- a/test/com/google/javascript/jscomp/NewTypeInferenceES5OrLowerTest.java
+++ b/test/com/google/javascript/jscomp/NewTypeInferenceES5OrLowerTest.java
@@ -16150,6 +16150,47 @@ public void testIObjectInheritance() {
         "function f(/** !Foo */ x, /** string */ s) {",
         "  x[s];",
         "}"));
+
+    typeCheck(LINE_JOINER.join(
+        "/** @interface @extends {IObject<string, string>} */",
+        "function Int1() {}",
+        "/** @interface @extends {IObject<string, number>} */",
+        "function Int2() {}",
+        "/**",
+        " * @constructor",
+        " * @implements {Int1}",
+        " * @implements {Int2}",
+        " */",
+        "function Foo() {}"),
+        GlobalTypeInfo.SUPER_INTERFACES_HAVE_INCOMPATIBLE_PROPERTIES);
+
+    typeCheck(LINE_JOINER.join(
+        "/** @interface @extends {IObject<string, string>} */",
+        "function Int1() {}",
+        "/** @interface @extends {IObject<string, number>} */",
+        "function Int2() {}",
+        "/**",
+        " * @constructor",
+        " * @implements {Int1}",
+        " * @implements {Int2}",
+        " */",
+        "function Foo() {}",
+        // Tests that we don't crash on property accesses of bad IObjects
+        "var /** null */ n = (new Foo)['asdf'+'asdf'];"),
+        GlobalTypeInfo.SUPER_INTERFACES_HAVE_INCOMPATIBLE_PROPERTIES);
+
+    typeCheck(LINE_JOINER.join(
+        "/** @interface @extends {IObject<function(number), number>} */",
+        "function Int1() {}",
+        "/** @interface @extends {IObject<function(string), number>} */",
+        "function Int2() {}",
+        "/**",
+        " * @constructor",
+        " * @implements {Int1}",
+        " * @implements {Int2}",
+        " */",
+        "function Foo() {}"),
+        GlobalTypeInfo.SUPER_INTERFACES_HAVE_INCOMPATIBLE_PROPERTIES);
   }
 
   public void testDontWarnForMissingReturnOnInfiniteLoop() {