diff --git a/mysql-test/main/mdev_39933.result b/mysql-test/main/mdev_39933.result
new file mode 100644
index 0000000000000..8f49ee48e4668
--- /dev/null
+++ b/mysql-test/main/mdev_39933.result
@@ -0,0 +1,43 @@
+#
+# MDEV-39933: Incorrect result of JSON_NORMALIZE on invalid json data
+#
+# JSON with trailing literal junk (no NUL) - should return NULL
+SELECT JSON_NORMALIZE('{"a":1}0junk');
+JSON_NORMALIZE('{"a":1}0junk')
+NULL
+# JSON_VALID correctly rejects JSON with embedded NUL byte + trailing junk
+SELECT JSON_VALID(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1));
+JSON_VALID(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1))
+0
+# JSON_NORMALIZE should also return NULL for invalid JSON with embedded NUL
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1));
+JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1))
+NULL
+# Additional case: just a NUL after valid JSON
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0)) USING latin1));
+JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0)) USING latin1))
+NULL
+# Valid JSON through conversion should still work
+SELECT JSON_NORMALIZE(CONVERT('{"a":1}' USING latin1));
+JSON_NORMALIZE(CONVERT('{"a":1}' USING latin1))
+{"a":1.0E0}
+# Multi-byte source charset (utf16)
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'x') USING utf16));
+JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'x') USING utf16))
+NULL
+# NUL embedded inside a JSON string value
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":"b', CHAR(0), 'c"}') USING latin1));
+JSON_NORMALIZE(CONVERT(CONCAT('{"a":"b', CHAR(0), 'c"}') USING latin1))
+NULL
+# JSON array with trailing NUL
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('[1,2,3]', CHAR(0), 'x') USING latin1));
+JSON_NORMALIZE(CONVERT(CONCAT('[1,2,3]', CHAR(0), 'x') USING latin1))
+NULL
+# Nested object - valid through conversion
+SELECT JSON_NORMALIZE(CONVERT('{"a":{"b":1}}' USING latin1));
+JSON_NORMALIZE(CONVERT('{"a":{"b":1}}' USING latin1))
+{"a":{"b":1.0E0}}
+# Empty string
+SELECT JSON_NORMALIZE(CONVERT('' USING latin1));
+JSON_NORMALIZE(CONVERT('' USING latin1))
+NULL
diff --git a/mysql-test/main/mdev_39933.test b/mysql-test/main/mdev_39933.test
new file mode 100644
index 0000000000000..ce9b4d7da9abc
--- /dev/null
+++ b/mysql-test/main/mdev_39933.test
@@ -0,0 +1,33 @@
+--echo #
+--echo # MDEV-39933: Incorrect result of JSON_NORMALIZE on invalid json data
+--echo #
+
+--echo # JSON with trailing literal junk (no NUL) - should return NULL
+SELECT JSON_NORMALIZE('{"a":1}0junk');
+
+--echo # JSON_VALID correctly rejects JSON with embedded NUL byte + trailing junk
+SELECT JSON_VALID(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1));
+
+--echo # JSON_NORMALIZE should also return NULL for invalid JSON with embedded NUL
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'junk') USING latin1));
+
+--echo # Additional case: just a NUL after valid JSON
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0)) USING latin1));
+
+--echo # Valid JSON through conversion should still work
+SELECT JSON_NORMALIZE(CONVERT('{"a":1}' USING latin1));
+
+--echo # Multi-byte source charset (utf16)
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":1}', CHAR(0), 'x') USING utf16));
+
+--echo # NUL embedded inside a JSON string value
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('{"a":"b', CHAR(0), 'c"}') USING latin1));
+
+--echo # JSON array with trailing NUL
+SELECT JSON_NORMALIZE(CONVERT(CONCAT('[1,2,3]', CHAR(0), 'x') USING latin1));
+
+--echo # Nested object - valid through conversion
+SELECT JSON_NORMALIZE(CONVERT('{"a":{"b":1}}' USING latin1));
+
+--echo # Empty string
+SELECT JSON_NORMALIZE(CONVERT('' USING latin1));
diff --git a/strings/json_normalize.c b/strings/json_normalize.c
index 81575f07069bc..257f0e41fe499 100644
--- a/strings/json_normalize.c
+++ b/strings/json_normalize.c
@@ -1030,15 +1030,14 @@ json_normalize(DYNAMIC_STRING *result,
     if (!s_utf8)
       return 1;
     memset(s_utf8, 0x00, in_size);
-    my_convert(s_utf8, (uint32)in_size, &my_charset_utf8mb4_bin,
-               s, (uint32)size, cs, &convert_err);
+    in_size= my_convert(s_utf8, (uint32)in_size, &my_charset_utf8mb4_bin,
+                       s, (uint32)size, cs, &convert_err);
     if (convert_err)
     {
        my_free(s_utf8);
        return 1;
     }
     in= s_utf8;
-    in_size= strlen(s_utf8);
   }