diff --git a/src/boot/efi/efi-string.c b/src/boot/efi/efi-string.c
index 87beb852860..aee98a81e1a 100644
--- a/src/boot/efi/efi-string.c
+++ b/src/boot/efi/efi-string.c
@@ -41,12 +41,13 @@ DEFINE_STRNLEN(char16_t, strnlen16);
                 (_c >= 'A' && _c <= 'Z') ? _c + ('a' - 'A') : _c; \
         })
 
-#define DEFINE_STRTOLOWER(type, name)     \
-        void name(type *s) {              \
-                if (!s)                   \
-                        return;           \
-                for (; *s; s++)           \
-                        *s = TOLOWER(*s); \
+#define DEFINE_STRTOLOWER(type, name)                \
+        type* name(type *s) {                        \
+                if (!s)                              \
+                        return NULL;                 \
+                for (type *p = s; *p; p++)           \
+                        *p = TOLOWER(*p);            \
+                return s;                            \
         }
 
 DEFINE_STRTOLOWER(char, strtolower8);
diff --git a/src/boot/efi/efi-string.h b/src/boot/efi/efi-string.h
index c7c49a07aa3..9a0f89d904c 100644
--- a/src/boot/efi/efi-string.h
+++ b/src/boot/efi/efi-string.h
@@ -23,8 +23,8 @@ static inline size_t strsize16(const char16_t *s) {
         return s ? (strlen16(s) + 1) * sizeof(*s) : 0;
 }
 
-void strtolower8(char *s);
-void strtolower16(char16_t *s);
+char* strtolower8(char *s);
+char16_t* strtolower16(char16_t *s);
 
 int strncmp8(const char *s1, const char *s2, size_t n);
 int strncmp16(const char16_t *s1, const char16_t *s2, size_t n);