diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 6cf1b50e1f9279..1ae24f61cf2a23 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -2662,9 +2662,13 @@ pub fn (s string) camel_to_snake() string {
 		}
 		lower_first_c, lower_second_c
 	} else {
-		lower_first_c := s[0]
-		second_c := if s[1].is_capital() { u8(`_`) } else { s[1] }
-		lower_first_c, second_c
+		first_c := s[0]
+		second_c := if s[1].is_capital() {
+			if first_c == `_` { s[1] + 32 } else { u8(`_`) }
+		} else {
+			s[1]
+		}
+		first_c, second_c
 	}
 	unsafe {
 		b[0] = first_char
diff --git a/vlib/builtin/string_test.v b/vlib/builtin/string_test.v
index 0ecb67c853bb0f..b9cf2dcf912c77 100644
--- a/vlib/builtin/string_test.v
+++ b/vlib/builtin/string_test.v
@@ -1529,6 +1529,7 @@ fn test_camel_to_snake() {
 	assert 'BBaa'.camel_to_snake() == 'b_baa'
 	assert 'aa_BB'.camel_to_snake() == 'aa_bb'
 	assert 'JVM_PUBLIC_ACC'.camel_to_snake() == 'jvm_public_acc'
+	assert '_ISspace'.camel_to_snake() == '_i_sspace'
 }
 
 fn test_snake_to_camel() {