diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 32bf6ab30d..437a6e12df 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -805,9 +805,9 @@ func EqualFold(s, t []byte) bool {
 			tr, sr = sr, tr
 		}
 		// Fast check for ASCII.
-		if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
-			// ASCII, and sr is upper case.  tr must be lower case.
-			if tr == sr+'a'-'A' {
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
 				continue
 			}
 			return false
diff --git a/src/strings/strings.go b/src/strings/strings.go
index b0a53fdefd..45345e0088 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -908,9 +908,9 @@ func EqualFold(s, t string) bool {
 			tr, sr = sr, tr
 		}
 		// Fast check for ASCII.
-		if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
-			// ASCII, and sr is upper case.  tr must be lower case.
-			if tr == sr+'a'-'A' {
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
 				continue
 			}
 			return false
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index d8fcb62a87..876f06c674 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -1379,6 +1379,8 @@ var EqualFoldTests = []struct {
 	{"abcdefghijK", "abcdefghij\u212A", true},
 	{"abcdefghijkz", "abcdefghij\u212Ay", false},
 	{"abcdefghijKz", "abcdefghij\u212Ay", false},
+	{"1", "2", false},
+	{"utf-8", "US-ASCII", false},
 }
 
 func TestEqualFold(t *testing.T) {
@@ -1392,6 +1394,16 @@ func TestEqualFold(t *testing.T) {
 	}
 }
 
+func BenchmarkEqualFold(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tt := range EqualFoldTests {
+			if out := EqualFold(tt.s, tt.t); out != tt.out {
+				b.Fatal("wrong result")
+			}
+		}
+	}
+}
+
 var CountTests = []struct {
 	s, sep string
 	num    int