diff --git a/src/core/include/mp-units/bits/external/hacks.h b/src/core/include/mp-units/bits/external/hacks.h
index 7e3c13c6..48dc6927 100644
--- a/src/core/include/mp-units/bits/external/hacks.h
+++ b/src/core/include/mp-units/bits/external/hacks.h
@@ -102,10 +102,6 @@
 
 #endif
 
-#ifndef MP_UNITS_USE_FMTLIB
-#define MP_UNITS_USE_FMTLIB 1
-#endif
-
 #if MP_UNITS_USE_FMTLIB
 
 MP_UNITS_DIAGNOSTIC_PUSH