diff --git a/src/eval.c b/src/eval.c
index 1471568..cf908d1 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -628,6 +628,9 @@
 static void f_invert(typval_T *argvars, typval_T *rettv);
 static void f_isdirectory(typval_T *argvars, typval_T *rettv);
 static void f_islocked(typval_T *argvars, typval_T *rettv);
+#if defined(FEAT_FLOAT) && defined(HAVE_MATH_H)
+static void f_isnan(typval_T *argvars, typval_T *rettv);
+#endif
 static void f_items(typval_T *argvars, typval_T *rettv);
 #ifdef FEAT_JOB
 # ifdef FEAT_CHANNEL
@@ -8320,6 +8323,9 @@
     {"invert",		1, 1, f_invert},
     {"isdirectory",	1, 1, f_isdirectory},
     {"islocked",	1, 1, f_islocked},
+#if defined(FEAT_FLOAT) && defined(HAVE_MATH_H)
+    {"isnan",		1, 1, f_isnan},
+#endif
     {"items",		1, 1, f_items},
 #ifdef FEAT_JOB
 # ifdef FEAT_CHANNEL
@@ -14740,6 +14746,18 @@
     clear_lval(&lv);
 }
 
+#if defined(FEAT_FLOAT) && defined(HAVE_MATH_H)
+/*
+ * "isnan()" function
+ */
+    static void
+f_isnan(typval_T *argvars, typval_T *rettv)
+{
+    rettv->vval.v_number = argvars[0].v_type == VAR_FLOAT
+					    && isnan(argvars[0].vval.v_float);
+}
+#endif
+
 static void dict_list(typval_T *argvars, typval_T *rettv, int what);
 
 /*