diff --git a/core/strings/strings.odin b/core/strings/strings.odin
index 6ace03b48..9179a9046 100644
--- a/core/strings/strings.odin
+++ b/core/strings/strings.odin
@@ -479,7 +479,7 @@ last_index :: proc(s, substr: string) -> int {
 	return -1
 }
 
-
+// index_any returns the index of the first char of `chars` found in `s`. -1 if not found.
 index_any :: proc(s, chars: string) -> int {
 	if chars == "" {
 		return -1
@@ -504,8 +504,8 @@ index_any :: proc(s, chars: string) -> int {
 		}
 	}
 
-	for c, i in chars {
-		if index_rune(chars, c) >= 0 {
+	for c in chars {
+		if i := index_rune(s, c); i >= 0 {
 			return i
 		}
 	}
diff --git a/tests/core/strings/test_core_strings.odin b/tests/core/strings/test_core_strings.odin
new file mode 100644
index 000000000..c9da5a688
--- /dev/null
+++ b/tests/core/strings/test_core_strings.odin
@@ -0,0 +1,29 @@
+package test_core_image
+
+import "core:strings"
+import "core:testing"
+
+@test
+test_index_any_small_string_not_found :: proc(t: ^testing.T) {
+    index := strings.index_any(".", "/:\"")
+    testing.log(t, index)
+    testing.expect(t, index == -1, "index_any should be negative")
+}
+
+@test
+test_index_any_larger_string_not_found :: proc(t: ^testing.T) {
+    index := strings.index_any("aaaaaaaa.aaaaaaaa", "/:\"")
+    testing.expect(t, index == -1, "index_any should be negative")
+}
+
+@test
+test_index_any_small_string_found :: proc(t: ^testing.T) {
+    index := strings.index_any(".", "/:.\"")
+    testing.expect(t, index == 0, "index_any should be 0")
+}
+
+@test
+test_index_any_larger_string_found :: proc(t: ^testing.T) {
+    index := strings.index_any("aaaaaaaa:aaaaaaaa", "/:\"")
+    testing.expect(t, index == 8, "index_any should be 8")
+}