diff --git a/vm.go b/vm.go
index 2e0240b..74b7742 100644
--- a/vm.go
+++ b/vm.go
@@ -536,6 +536,7 @@ func (v *VM) run() {
 				v.sp++
 			default:
 				v.err = fmt.Errorf("not indexable: %s", left.TypeName())
+				return
 			}
 		case parser.OpCall:
 			numArgs := int(v.curInsts[v.ip+1])
diff --git a/vm_test.go b/vm_test.go
index 49c93de..55a9289 100644
--- a/vm_test.go
+++ b/vm_test.go
@@ -3638,6 +3638,7 @@ func TestSliceIndex(t *testing.T) {
 	expectError(t, `undefined[:1]`, nil, "Runtime Error: not indexable")
 	expectError(t, `123[-1:2]`, nil, "Runtime Error: not indexable")
 	expectError(t, `{}[:]`, nil, "Runtime Error: not indexable")
+	expectError(t, `a := 123[-1:2] ; a += 1`, nil, "Runtime Error: not indexable")
 }
 
 func expectRun(