diff --git a/pandas/_libs/tslib.pyx b/pandas/_libs/tslib.pyx
index 3c5854602df53..2352020b654cc 100644
--- a/pandas/_libs/tslib.pyx
+++ b/pandas/_libs/tslib.pyx
@@ -94,10 +94,15 @@ def _test_parse_iso8601(ts: str):
         _TSObject obj
         int out_local = 0, out_tzoffset = 0
         NPY_DATETIMEUNIT out_bestunit
+        bint failed
 
     obj = _TSObject()
 
-    string_to_dts(ts, &obj.dts, &out_bestunit, &out_local, &out_tzoffset, True)
+    failed = string_to_dts(
+        ts, &obj.dts, &out_bestunit, &out_local, &out_tzoffset, False
+    )
+    if failed:
+        raise ValueError("string_to_dts failed")
     try:
         obj.value = npy_datetimestruct_to_datetime(NPY_FR_ns, &obj.dts)
     except OverflowError as err:
diff --git a/pandas/_libs/tslibs/np_datetime.pxd b/pandas/_libs/tslibs/np_datetime.pxd
index 3e5654b70cd92..6d7ad4bae8a1d 100644
--- a/pandas/_libs/tslibs/np_datetime.pxd
+++ b/pandas/_libs/tslibs/np_datetime.pxd
@@ -91,7 +91,7 @@ cdef int string_to_dts(
     bint want_exc,
     str format = *,
     bint exact = *
-) except? -1
+)
 
 cdef NPY_DATETIMEUNIT get_unit_from_dtype(cnp.dtype dtype)
 
diff --git a/pandas/_libs/tslibs/np_datetime.pyx b/pandas/_libs/tslibs/np_datetime.pyx
index 1b7f04fe17238..5853879f98679 100644
--- a/pandas/_libs/tslibs/np_datetime.pyx
+++ b/pandas/_libs/tslibs/np_datetime.pyx
@@ -346,7 +346,7 @@ cdef int string_to_dts(
     bint want_exc,
     str format=None,
     bint exact=True,
-) except? -1:
+):
     cdef:
         Py_ssize_t length
         const char* buf