diff --git a/Misc/NEWS.d/next/Library/2025-06-16-15-03-03.gh-issue-135561.mJCN8D.rst b/Misc/NEWS.d/next/Library/2025-06-16-15-03-03.gh-issue-135561.mJCN8D.rst
new file mode 100644
index 00000000000000..ee743f161138e6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-16-15-03-03.gh-issue-135561.mJCN8D.rst
@@ -0,0 +1,2 @@
+Fix a crash on DEBUG builds when an HACL* HMAC routine fails. Patch by
+Bénédikt Tran.
diff --git a/Modules/hmacmodule.c b/Modules/hmacmodule.c
index b404d5732ec857..9c92d9d145f5a3 100644
--- a/Modules/hmacmodule.c
+++ b/Modules/hmacmodule.c
@@ -493,36 +493,41 @@ narrow_hmac_hash_kind(hmacmodule_state *state, HMAC_Hash_Kind kind)
 static int
 _hacl_convert_errno(hacl_errno_t code, PyObject *algorithm)
 {
+    assert(PyGILState_GetThisThreadState() != NULL);
+    if (code == Hacl_Streaming_Types_Success) {
+        return 0;
+    }
+
+    PyGILState_STATE gstate = PyGILState_Ensure();
     switch (code) {
-        case Hacl_Streaming_Types_Success: {
-            return 0;
-        }
         case Hacl_Streaming_Types_InvalidAlgorithm: {
             // only makes sense if an algorithm is known at call time
             assert(algorithm != NULL);
             assert(PyUnicode_CheckExact(algorithm));
             PyErr_Format(PyExc_ValueError, "invalid algorithm: %U", algorithm);
-            return -1;
+            break;
         }
         case Hacl_Streaming_Types_InvalidLength: {
             PyErr_SetString(PyExc_ValueError, "invalid length");
-            return -1;
+            break;
         }
         case Hacl_Streaming_Types_MaximumLengthExceeded: {
             PyErr_SetString(PyExc_OverflowError, "maximum length exceeded");
-            return -1;
+            break;
         }
         case Hacl_Streaming_Types_OutOfMemory: {
             PyErr_NoMemory();
-            return -1;
+            break;
         }
         default: {
             PyErr_Format(PyExc_RuntimeError,
                          "HACL* internal routine failed with error code: %d",
                          code);
-            return -1;
+            break;
         }
     }
+    PyGILState_Release(gstate);
+    return -1;
 }
 
 /*