diff --git a/Misc/NEWS b/Misc/NEWS
index 8015894..e384a92 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #5939: Add additional runtime checking to ensure a valid capsule
+  in Modules/_ctypes/callproc.c.
+
 - Issue #7309: Fix unchecked attribute access when converting
   UnicodeEncodeError, UnicodeDecodeError, and UnicodeTranslateError to
   strings.
diff --git a/Modules/_ctypes/callproc.c b/Modules/_ctypes/callproc.c
index f5eaa0d..2a75fb1 100644
--- a/Modules/_ctypes/callproc.c
+++ b/Modules/_ctypes/callproc.c
@@ -139,8 +139,14 @@
 			return NULL;
 	}
 	errobj = PyDict_GetItem(dict, error_object_name);
-	if (errobj)
+	if (errobj) {
+		if (!PyCapsule_IsValid(errobj, CTYPES_CAPSULE_NAME_PYMEM)) {
+			PyErr_SetString(PyExc_RuntimeError,
+				"ctypes.error_object is an invalid capsule");
+			return NULL;
+		}
 		Py_INCREF(errobj);
+	}
 	else {
 		void *space = PyMem_Malloc(sizeof(int) * 2);
 		if (space == NULL)