diff --git a/code/rewrite.go b/code/rewrite.go
index fcf53cf..12365a7 100644
--- a/code/rewrite.go
+++ b/code/rewrite.go
@@ -112,7 +112,11 @@ func ToComments(wdst io.Writer, rsrc io.Reader) ([]*Failpoint, error) {
 
 			ws = strings.Split(l, "i")[0]
 			n := strings.Split(strings.Split(l, "__fp_")[1], ".")[0]
-			t := strings.Split(strings.Split(l, ".(")[1], ")")[0]
+			parts := strings.Split(l, ".(")
+			if len(parts) < 2 {
+				return fps, fmt.Errorf("failpoint %q header is missing its type assertion, the generated code may have been reformatted: %q", n, lTrim)
+			}
+			t := strings.Split(parts[1], ")")[0]
 			dst.WriteString(ws + pfx + " var " + n + " " + t + "\n")
 			if !strings.Contains(l, "; goto __nomock") {
 				// not single liner
diff --git a/code/rewrite_test.go b/code/rewrite_test.go
index 07dc685..9925e90 100644
--- a/code/rewrite_test.go
+++ b/code/rewrite_test.go
@@ -124,3 +124,16 @@ func TestToComment(t *testing.T) {
 		require.Equalf(t, len(fps), ex.wfps, "%d: got %d failpoints but expected %d", i, len(fps), ex.wfps)
 	}
 }
+
+func TestToCommentsReformattedHeader(t *testing.T) {
+	// gofmt splits the header across lines:
+	// if vTest, __fpErr := __fp_Test.Acquire(); __fpErr == nil {
+	//   Test, __fpTypeOK := vTest.(int); ...
+	// The second line contains ".(" but not the first, so strings.Split(l, ".(")[1] panics.
+	reformatted := "if vTest, __fpErr := __fp_Test.Acquire(); __fpErr == nil {\n\tTest, __fpTypeOK := vTest.(int)\n\tfmt.Println(Test)\n}\n"
+	dst := bytes.NewBuffer(nil)
+	src := strings.NewReader(reformatted)
+	_, err := ToComments(dst, src)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "type assertion")
+}