diff --git a/Modules/_testcapi/watchers.c b/Modules/_testcapi/watchers.c
index 6d061bb8d51040..5a756a87c15fe9 100644
--- a/Modules/_testcapi/watchers.c
+++ b/Modules/_testcapi/watchers.c
@@ -364,7 +364,7 @@ add_code_watcher(PyObject *self, PyObject *which_watcher)
         watcher_id = PyCode_AddWatcher(error_code_event_handler);
     }
     else {
-        PyErr_Format(PyExc_ValueError, "invalid watcher %d", which_l);
+        PyErr_Format(PyExc_ValueError, "invalid watcher %ld", which_l);
         return NULL;
     }
     if (watcher_id < 0) {
@@ -673,7 +673,7 @@ add_context_watcher(PyObject *self, PyObject *which_watcher)
     assert(PyLong_Check(which_watcher));
     long which_l = PyLong_AsLong(which_watcher);
     if (which_l < 0 || which_l >= (long)Py_ARRAY_LENGTH(callbacks)) {
-        PyErr_Format(PyExc_ValueError, "invalid watcher %d", which_l);
+        PyErr_Format(PyExc_ValueError, "invalid watcher %ld", which_l);
         return NULL;
     }
     int watcher_id = PyContext_AddWatcher(callbacks[which_l]);
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index f6bd48b8d2a848..53f630832f8bbc 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -116,8 +116,8 @@ test_sizeof_c_types(PyObject *self, PyObject *Py_UNUSED(ignored))
     do { \
         if (EXPECTED != sizeof(TYPE)) { \
             PyErr_Format(get_testerror(self),               \
-                         "sizeof(%s) = %u instead of %u",   \
-                         #TYPE, sizeof(TYPE), EXPECTED);    \
+                         "sizeof(%s) = %zu instead of %u",   \
+                         #TYPE, sizeof(TYPE), (unsigned)(EXPECTED));    \
             return (PyObject*)NULL; \
         } \
     } while (0)
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 76697ee9c41a0e..b2b4e4ce1d2bd9 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -144,14 +144,14 @@ test_bswap(PyObject *self, PyObject *Py_UNUSED(args))
     uint16_t u16 = _Py_bswap16(UINT16_C(0x3412));
     if (u16 != UINT16_C(0x1234)) {
         PyErr_Format(PyExc_AssertionError,
-                     "_Py_bswap16(0x3412) returns %u", u16);
+                     "_Py_bswap16(0x3412) returns %d", u16);
         return NULL;
     }
 
     uint32_t u32 = _Py_bswap32(UINT32_C(0x78563412));
     if (u32 != UINT32_C(0x12345678)) {
         PyErr_Format(PyExc_AssertionError,
-                     "_Py_bswap32(0x78563412) returns %lu", u32);
+                     "_Py_bswap32(0x78563412) returns %u", u32);
         return NULL;
     }
 
@@ -430,7 +430,7 @@ test_edit_cost(PyObject *self, PyObject *Py_UNUSED(args))
 
 static int
 check_bytes_find(const char *haystack0, const char *needle0,
-                 int offset, Py_ssize_t expected)
+                 Py_ssize_t offset, Py_ssize_t expected)
 {
     Py_ssize_t len_haystack = strlen(haystack0);
     Py_ssize_t len_needle = strlen(needle0);
@@ -848,7 +848,7 @@ get_interp_settings(PyObject *self, PyObject *args)
     }
     else {
         PyErr_Format(PyExc_NotImplementedError,
-                     "%zd", interpid);
+                     "%d", interpid);
         return NULL;
     }
     assert(interp != NULL);