diff --git a/bytesconv.go b/bytesconv.go
index 4e36d1d7b3..a44e226221 100644
--- a/bytesconv.go
+++ b/bytesconv.go
@@ -73,15 +73,11 @@ func ParseIPv4(dst net.IP, ipStr []byte) (net.IP, error) {
 	if len(ipStr) == 0 {
 		return dst, errEmptyIPStr
 	}
-	if len(dst) < net.IPv4len {
+	if len(dst) < net.IPv4len || len(dst) > net.IPv4len {
 		dst = make([]byte, net.IPv4len)
 	}
 	copy(dst, net.IPv4zero)
 	dst = dst.To4()
-	if dst == nil {
-		// developer sanity-check
-		panic("BUG: dst must not be nil")
-	}
 
 	b := ipStr
 	for i := 0; i < 3; i++ {
diff --git a/bytesconv_test.go b/bytesconv_test.go
index 54e5e20022..d5ca5e4341 100644
--- a/bytesconv_test.go
+++ b/bytesconv_test.go
@@ -59,21 +59,27 @@ func testAppendHTMLEscape(t *testing.T, s, expectedS string) {
 func TestParseIPv4(t *testing.T) {
 	t.Parallel()
 
-	testParseIPv4(t, "0.0.0.0", true)
-	testParseIPv4(t, "255.255.255.255", true)
-	testParseIPv4(t, "123.45.67.89", true)
+	testParseIPv4(t, net.IP{0}, "0.0.0.0", true)
+	testParseIPv4(t, nil, "0.0.0.0", true)
+	testParseIPv4(t, net.IP{0, 0, 0, 0, 0}, "0.0.0.0", true)
+	testParseIPv4(t, nil, "255.255.255.255", true)
+	testParseIPv4(t, nil, "123.45.67.89", true)
 
 	// ipv6 shouldn't work
-	testParseIPv4(t, "2001:4860:0:2001::68", false)
+	testParseIPv4(t, nil, "2001:4860:0:2001::68", false)
 
 	// invalid ip
-	testParseIPv4(t, "foobar", false)
-	testParseIPv4(t, "1.2.3", false)
-	testParseIPv4(t, "123.456.789.11", false)
+	testParseIPv4(t, nil, "", false)
+	testParseIPv4(t, nil, "foobar", false)
+	testParseIPv4(t, nil, "1.2.3", false)
+	testParseIPv4(t, nil, "123.456.789.11", false)
+	testParseIPv4(t, nil, "b.1.2.3", false)
+	testParseIPv4(t, nil, "1.2.3.b", false)
+	testParseIPv4(t, nil, "1.2.3.456", false)
 }
 
-func testParseIPv4(t *testing.T, ipStr string, isValid bool) {
-	ip, err := ParseIPv4(nil, []byte(ipStr))
+func testParseIPv4(t *testing.T, dst net.IP, ipStr string, isValid bool) {
+	ip, err := ParseIPv4(dst, []byte(ipStr))
 	if isValid {
 		if err != nil {
 			t.Fatalf("unexpected error when parsing ip %q: %v", ipStr, err)