diff --git a/racket/src/configure b/racket/src/configure
index dbc52ea48d..21aecfa370 100755
--- a/racket/src/configure
+++ b/racket/src/configure
@@ -6583,6 +6583,11 @@ $as_echo "#define MZ_NO_EXTFLONUMS 1" >>confdefs.h
   fi
 fi
 
+if test "${enable_extflonum}" = "yes" ; then
+  $as_echo "#define MZ_INSIST_EXTFLONUMS 1" >>confdefs.h
+
+fi
+
 ############## libffi ################
 
 # Depends on pthread on some platforms
diff --git a/racket/src/racket/configure.ac b/racket/src/racket/configure.ac
index 46b76c953f..bff753b8ed 100644
--- a/racket/src/racket/configure.ac
+++ b/racket/src/racket/configure.ac
@@ -1475,6 +1475,10 @@ if test "${enable_extflonum}" = "default" ; then
   fi
 fi
 
+if test "${enable_extflonum}" = "yes" ; then
+  AC_DEFINE(MZ_INSIST_EXTFLONUMS)
+fi
+
 ############## libffi ################
 
 # Depends on pthread on some platforms
diff --git a/racket/src/racket/include/scheme.h b/racket/src/racket/include/scheme.h
index 1ede3d0c9b..4381de52e2 100644
--- a/racket/src/racket/include/scheme.h
+++ b/racket/src/racket/include/scheme.h
@@ -93,6 +93,9 @@ typedef struct {
 typedef long double mz_long_double;
 # endif
 #else
+# ifdef MZ_INSIST_EXTFLONUMS
+#  error "cannot support extflonums; you may need to adjust compiler options"
+# endif
 typedef double mz_long_double;
 #endif
 
diff --git a/racket/src/racket/mzconfig.h.in b/racket/src/racket/mzconfig.h.in
index 154363d47e..8987178abf 100644
--- a/racket/src/racket/mzconfig.h.in
+++ b/racket/src/racket/mzconfig.h.in
@@ -95,6 +95,9 @@ typedef unsigned long uintptr_t;
 /* To disable extflonums when they would otherwise work: */
 #undef MZ_NO_EXTFLONUMS
 
+/* Extflonums are specifically requested (so complain if not supported): */
+#undef MZ_INSIST_EXTFLONUMS
+
 /* Library subpath */
 #undef SPLS_SUFFIX