diff --git a/sem/typecheck/typecheck.go b/sem/typecheck/typecheck.go
index 15775d2..a152090 100644
--- a/sem/typecheck/typecheck.go
+++ b/sem/typecheck/typecheck.go
@@ -189,8 +189,7 @@ func check(file *ast.File, exprTypes map[ast.Expr]types.Type) error {
 			if !ok {
 				panic(fmt.Sprintf("unable to locate type of expression %v", n.Index))
 			}
-			num, ok := indexType.(types.Numerical)
-			if !ok || !num.IsNumerical() {
+			if !types.IsInteger(indexType) {
 				return errors.Newf(n.Index.Start(), "invalid array index; expected integer, got %q", indexType)
 			}
 		default:
diff --git a/types/types.go b/types/types.go
index 1279f91..f50a489 100644
--- a/types/types.go
+++ b/types/types.go
@@ -147,6 +147,18 @@ func IsVoid(t Type) bool {
 	return false
 }
 
+// IsInteger reports whether the given type is an integer (i.e. "int" or
+// "char").
+func IsInteger(t Type) bool {
+	if t, ok := t.(*Basic); ok {
+		switch t.Kind {
+		case Int, Char:
+			return true
+		}
+	}
+	return false
+}
+
 // IsNumerical reports whether the given type is numerical.
 func (t *Basic) IsNumerical() bool {
 	switch t.Kind {