diff --git a/promql/parse.go b/promql/parse.go
index aac36cedf..c1bc1fd7d 100644
--- a/promql/parse.go
+++ b/promql/parse.go
@@ -321,6 +321,8 @@ func (p *parser) expectOneOf(exp1, exp2 itemType, context string) item {
 	return token
 }
 
+var errUnexpected = fmt.Errorf("unexpected error")
+
 // recover is the handler that turns panics into returns from the top level of Parse.
 func (p *parser) recover(errp *error) {
 	e := recover()
@@ -331,7 +333,7 @@ func (p *parser) recover(errp *error) {
 			buf = buf[:runtime.Stack(buf, false)]
 
 			log.Errorf("parser panic: %v\n%s", e, buf)
-			*errp = fmt.Errorf("unexpected error")
+			*errp = errUnexpected
 		} else {
 			*errp = e.(error)
 		}
diff --git a/promql/parse_test.go b/promql/parse_test.go
index 7f938c3dc..3fc1df873 100644
--- a/promql/parse_test.go
+++ b/promql/parse_test.go
@@ -973,6 +973,12 @@ func TestParseExpressions(t *testing.T) {
 		parser := newParser(test.input)
 
 		expr, err := parser.parseExpr()
+
+		// Unexpected errors are always caused by a bug.
+		if err == errUnexpected {
+			t.Fatalf("unexpected error occurred")
+		}
+
 		if !test.fail && err != nil {
 			t.Errorf("error in input '%s'", test.input)
 			t.Fatalf("could not parse: %s", err)
@@ -1219,6 +1225,19 @@ var testStatement = []struct {
 		`,
 		fail: true,
 	},
+	// Fuzzing regression tests.
+	{
+		input: `I=-/`,
+		fail:  true,
+	},
+	{
+		input: `I=3E8/-=`,
+		fail:  true,
+	},
+	{
+		input: `M=-=-0-0`,
+		fail:  true,
+	},
 }
 
 func TestParseStatements(t *testing.T) {
@@ -1226,6 +1245,12 @@ func TestParseStatements(t *testing.T) {
 		parser := newParser(test.input)
 
 		stmts, err := parser.parseStmts()
+
+		// Unexpected errors are always caused by a bug.
+		if err == errUnexpected {
+			t.Fatalf("unexpected error occurred")
+		}
+
 		if !test.fail && err != nil {
 			t.Errorf("error in input: \n\n%s\n", test.input)
 			t.Fatalf("could not parse: %s", err)
@@ -1353,6 +1378,12 @@ func TestParseSeries(t *testing.T) {
 		parser.lex.seriesDesc = true
 
 		metric, vals, err := parser.parseSeriesDesc()
+
+		// Unexpected errors are always caused by a bug.
+		if err == errUnexpected {
+			t.Fatalf("unexpected error occurred")
+		}
+
 		if !test.fail && err != nil {
 			t.Errorf("error in input: \n\n%s\n", test.input)
 			t.Fatalf("could not parse: %s", err)
@@ -1385,8 +1416,8 @@ func TestRecoverRuntime(t *testing.T) {
 	var a []int
 	a[123] = 1
 
-	if err.Error() != "unexpected error" {
-		t.Fatalf("wrong error message: %q, expected %q", err, "unexpected error")
+	if err != errUnexpected {
+		t.Fatalf("wrong error message: %q, expected %q", err, errUnexpected)
 	}
 }