diff --git a/src/core/wee-string.c b/src/core/wee-string.c
index 783bd63e3..1ff0c53b9 100644
--- a/src/core/wee-string.c
+++ b/src/core/wee-string.c
@@ -2954,7 +2954,7 @@ string_base16_decode (const char *from, char *to)
     unsigned char value;
 
     if (!from || !to)
-        return 0;
+        return -1;
 
     count = 0;
 
@@ -3251,7 +3251,7 @@ string_base64_decode (const char *from, char *to)
         "@ABCDEFGHIJKLMNOPQRSTUVW$$$$$$XYZ[\\]^_`abcdefghijklmnopq";
 
     if (!from || !to)
-        return 0;
+        return -1;
 
     ptr_from = from;
     ptr_to = to;
diff --git a/tests/unit/core/test-core-string.cpp b/tests/unit/core/test-core-string.cpp
index 24801e29c..21dc894be 100644
--- a/tests/unit/core/test-core-string.cpp
+++ b/tests/unit/core/test-core-string.cpp
@@ -1705,9 +1705,9 @@ TEST(CoreString, Base16)
     }
 
     /* string_base16_decode */
-    LONGS_EQUAL(0, string_base16_decode (NULL, NULL));
-    LONGS_EQUAL(0, string_base16_decode (NULL, str));
-    LONGS_EQUAL(0, string_base16_decode ("", NULL));
+    LONGS_EQUAL(-1, string_base16_decode (NULL, NULL));
+    LONGS_EQUAL(-1, string_base16_decode (NULL, str));
+    LONGS_EQUAL(-1, string_base16_decode ("", NULL));
     LONGS_EQUAL(0, string_base16_decode ("", str));
     for (i = 0; str_base16[i][0]; i++)
     {
@@ -1828,9 +1828,9 @@ TEST(CoreString, Base64)
     }
 
     /* string_base64_decode */
-    LONGS_EQUAL(0, string_base64_decode (NULL, NULL));
-    LONGS_EQUAL(0, string_base64_decode (NULL, str));
-    LONGS_EQUAL(0, string_base64_decode ("", NULL));
+    LONGS_EQUAL(-1, string_base64_decode (NULL, NULL));
+    LONGS_EQUAL(-1, string_base64_decode (NULL, str));
+    LONGS_EQUAL(-1, string_base64_decode ("", NULL));
     LONGS_EQUAL(0, string_base64_decode ("", str));
     for (i = 0; str_base64[i][0]; i++)
     {