diff --git a/tests/suites/test_suite_base64.function b/tests/suites/test_suite_base64.function
index df63aea06f..5d8ed9bf9f 100644
--- a/tests/suites/test_suite_base64.function
+++ b/tests/suites/test_suite_base64.function
@@ -132,6 +132,14 @@ void mbedtls_base64_decode(char *src_string, char *dst_string, int result)
         TEST_EQUAL(correct_dst_len, len);
     }
 
+    /* Test dst=NULL with dlen!=0 (explicitly documented as supported) */
+    if (result == 0 && dst_size != 0) {
+        TEST_EQUAL(mbedtls_base64_decode(NULL, 42, &len,
+                                         src, src_len),
+                   MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL);
+        TEST_EQUAL(correct_dst_len, len);
+    }
+
 exit:
     mbedtls_free(src);
     mbedtls_free(dst);