diff --git a/src/PJ_robin.c b/src/PJ_robin.c
index 7514b32564..92aebfd32a 100644
--- a/src/PJ_robin.c
+++ b/src/PJ_robin.c
@@ -1,4 +1,5 @@
 #define PJ_LIB__
+#include "proj_internal.h"
 #include "proj.h"
 #include "projects.h"
 
@@ -80,7 +81,8 @@ static XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
     double dphi;
     (void) P;
 
-    i = (int)floor((dphi = fabs(lp.phi)) * C1);
+    dphi = fabs(lp.phi);
+    i = pj_is_nan(lp.phi) ? -1 : (int)floor(dphi * C1);
     if( i < 0 ){
         proj_errno_set(P, PJD_ERR_TOLERANCE_CONDITION);
         return xy;
@@ -115,7 +117,7 @@ static LP s_inverse (XY xy, PJ *P) {           /* Spheroidal, inverse */
         }
     } else { /* general problem */
         /* in Y space, reduce to table interval */
-        i = (int)floor(lp.phi * NODES);
+        i = pj_is_nan(lp.phi) ? -1 : (int)floor(lp.phi * NODES);
         if( i < 0 || i >= NODES ) {
             proj_errno_set(P, PJD_ERR_TOLERANCE_CONDITION);
             return lp;
diff --git a/src/nad_intr.c b/src/nad_intr.c
index b385410f58..dc245831ab 100644
--- a/src/nad_intr.c
+++ b/src/nad_intr.c
@@ -1,6 +1,8 @@
 /* Determine nad table correction value */
 #define PJ_LIB__
+#include "proj_internal.h"
 #include "projects.h"
+
 	LP
 nad_intr(LP t, struct CTABLE *ct) {
 	LP val, frct;
@@ -10,8 +12,11 @@ nad_intr(LP t, struct CTABLE *ct) {
 	long index;
 	int in;
 
-	indx.lam = (int)floor(t.lam /= ct->del.lam);
-	indx.phi = (int)floor(t.phi /= ct->del.phi);
+	t.lam /= ct->del.lam;
+	indx.lam = pj_is_nan(t.lam) ? 0 : (int)floor(t.lam);
+	t.phi /= ct->del.phi;
+	indx.phi = pj_is_nan(t.phi) ? 0 : (int)floor(t.phi);
+
 	frct.lam = t.lam - indx.lam;
 	frct.phi = t.phi - indx.phi;
 	val.lam = val.phi = HUGE_VAL;
diff --git a/src/pj_internal.c b/src/pj_internal.c
index 61905259f4..9cbbf20a5f 100644
--- a/src/pj_internal.c
+++ b/src/pj_internal.c
@@ -443,3 +443,15 @@ void proj_log_func (PJ_CONTEXT *ctx, void *app_data, PJ_LOG_FUNCTION logf) {
     if (0!=logf)
         ctx->logger = logf;
 }
+
+
+/*****************************************************************************/
+int pj_is_nan (double val) {
+/******************************************************************************
+    Returns 0 if not a NaN and non-zero if val is a NaN.
+
+    Provides an equivalent to isnan().
+******************************************************************************/
+    /* cppcheck-suppress duplicateExpression */
+    return val != val;
+}
diff --git a/src/proj_internal.h b/src/proj_internal.h
index 75893c33b2..b3843a59f8 100644
--- a/src/proj_internal.h
+++ b/src/proj_internal.h
@@ -130,6 +130,8 @@ void proj_fileapi_set (PJ *P, void *fileapi);
 const char * const *proj_get_searchpath(void);
 int    proj_get_path_count(void);
 
+int pj_is_nan (double val);
+
 #ifdef __cplusplus
 }
 #endif