Index: test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
===================================================================
--- test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
+++ test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
@@ -31,7 +31,9 @@
     errno = E2BIG; // something that message will never generate
     const std::error_category& e_cat1 = std::generic_category();
     const std::string msg = e_cat1.message(-1);
-    LIBCPP_ASSERT(msg == "Unknown error -1" || msg == "Unknown error");
+    // Exact message format varies by platform.
+    LIBCPP_ASSERT(msg == "Unknown error -1" || msg == "Unknown error" ||
+                  msg == "Unknown error: -1");
     assert(errno == E2BIG);
 }
 
Index: test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
===================================================================
--- test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
+++ test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
@@ -31,7 +31,9 @@
     errno = E2BIG; // something that message will never generate
     const std::error_category& e_cat1 = std::system_category();
     const std::string msg = e_cat1.message(-1);
-    LIBCPP_ASSERT(msg == "Unknown error -1" || msg == "Unknown error");
+    // Exact message format varries by platform.
+    LIBCPP_ASSERT(msg == "Unknown error -1" || msg == "Unknown error" ||
+                  msg == "Unknown error: -1");
     assert(errno == E2BIG);
 }