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diff --git a/lib/libc/stdlib/strfmon.3 b/lib/libc/stdlib/strfmon.3
index 20cc560d401d..d537d9d05fa7 100644
--- a/lib/libc/stdlib/strfmon.3
+++ b/lib/libc/stdlib/strfmon.3
@@ -22,7 +22,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd December 6, 2023
+.Dd November 24, 2025
.Dt STRFMON 3
.Os
.Sh NAME
@@ -182,6 +182,16 @@ function will fail if:
Conversion stopped due to lack of space in the buffer.
.It Bq Er EINVAL
The format string is invalid.
+.It Bq Er EINVAL
+The
+.Cm +
+flag was included in a conversion specification and the locale's
+.Va positive_sign
+and
+.Va negative_sign
+values would both be returned by
+.Xr localeconv 3
+as empty strings.
.It Bq Er ENOMEM
Not enough memory for temporary buffers.
.El
diff --git a/lib/libc/stdlib/strfmon.c b/lib/libc/stdlib/strfmon.c
index 230d194233f5..b19d9fc43369 100644
--- a/lib/libc/stdlib/strfmon.c
+++ b/lib/libc/stdlib/strfmon.c
@@ -171,7 +171,9 @@ literal:
flags &= ~(NEED_GROUPING);
continue;
case '+': /* use locale defined signs */
- if (flags & SIGN_POSN_USED)
+ if ((flags & SIGN_POSN_USED) ||
+ ((lc->positive_sign[0] == '\0') &&
+ (lc->negative_sign[0] == '\0')))
goto format_error;
flags |= (SIGN_POSN_USED | LOCALE_POSN);
continue;
diff --git a/lib/libc/tests/stdlib/strfmon_test.c b/lib/libc/tests/stdlib/strfmon_test.c
index 3c97d61ba82c..165ddcc2ab56 100644
--- a/lib/libc/tests/stdlib/strfmon_test.c
+++ b/lib/libc/tests/stdlib/strfmon_test.c
@@ -234,11 +234,11 @@ ATF_TC_BODY(strfmon_plus_or_parenthesis, tc)
if (setlocale(LC_MONETARY, "C") == NULL)
atf_tc_skip("unable to setlocale(): %s", tests[i].locale);
- /* ATF_CHECK_ERRNO(EINVAL, strfmon(actual, sizeof(actual) - 1,
- "[%+n] [%+n]", 123.45, -123.45)); XXX */
+ ATF_CHECK_ERRNO(EINVAL, strfmon(actual, sizeof(actual) - 1,
+ "[%+n] [%+n]", 123.45, -123.45));
- /* ATF_CHECK_ERRNO(EINVAL, strfmon(actual, sizeof(actual) - 1,
- "[%+i] [%+i]", 123.45, -123.45)); XXX */
+ ATF_CHECK_ERRNO(EINVAL, strfmon(actual, sizeof(actual) - 1,
+ "[%+i] [%+i]", 123.45, -123.45));
}
ATF_TC(strfmon_l);