diff --git a/src/fallback.cpp b/src/fallback.cpp
index c3ef3462f..cc12d53c9 100644
--- a/src/fallback.cpp
+++ b/src/fallback.cpp
@@ -255,10 +255,6 @@ int killpg(int pgr, int sig) {
 }
 #endif
 
-#ifndef HAVE_NAN
-double nan(char *tagp) { return 0.0 / 0.0; }
-#endif
-
 // Big hack to use our versions of wcswidth where we know them to be broken, which is
 // EVERYWHERE (https://github.com/fish-shell/fish-shell/issues/2199)
 #ifndef HAVE_BROKEN_WCWIDTH
diff --git a/src/fallback.h b/src/fallback.h
index 7a609b8dd..49120c901 100644
--- a/src/fallback.h
+++ b/src/fallback.h
@@ -125,8 +125,4 @@ char *fish_textdomain(const char *domainname);
 int killpg(int pgr, int sig);
 #endif
 
-#ifndef HAVE_NAN
-double nan(char *tagp);
-#endif
-
 #endif