diff --git a/libc/bionic/strerror_r.cpp b/libc/bionic/strerror_r.cpp
index e6a3975..646cc52 100644
--- a/libc/bionic/strerror_r.cpp
+++ b/libc/bionic/strerror_r.cpp
@@ -49,7 +49,7 @@
   if (error_name != NULL) {
     length = snprintf(buf, buf_len, "%s", error_name);
   } else {
-    length = snprintf(buf, buf_len, "Unknown error %u", error_number);
+    length = snprintf(buf, buf_len, "Unknown error %d", error_number);
   }
   if (length >= buf_len) {
     errno = ERANGE;
diff --git a/tests/string_test.cpp b/tests/string_test.cpp
index 71998d8..d55771c 100644
--- a/tests/string_test.cpp
+++ b/tests/string_test.cpp
@@ -39,7 +39,7 @@
   ASSERT_STREQ("Operation not permitted", strerror(1));
 
   // Invalid.
-  ASSERT_STREQ("Unknown error 4294967295", strerror(-1));
+  ASSERT_STREQ("Unknown error -1", strerror(-1));
   ASSERT_STREQ("Unknown error 1234", strerror(1234));
 }