--- a/js/src/dtoa.c
+++ b/js/src/dtoa.c
@@ -248,17 +248,17 @@ extern void *MALLOC(size_t);
 #ifdef KR_headers
 #define CONST /* blank */
 #else
 #define CONST const
 #endif
 #endif
 
 #if defined(IEEE_8087) + defined(IEEE_MC68k) + defined(VAX) + defined(IBM) != 1
-Exactly one of IEEE_8087, IEEE_MC68k, VAX, or IBM should be defined.
+#error "Exactly one of IEEE_8087, IEEE_MC68k, VAX, or IBM should be defined."
 #endif
 
 typedef union { double d; ULong L[2]; } U;
 
 #define dval(x) ((x).d)
 #ifdef IEEE_8087
 #define word0(x) ((x).L[1])
 #define word1(x) ((x).L[0])