diff --git a/src/confscreen.cpp b/src/confscreen.cpp
index 646f91e0..cdf6e518 100644
--- a/src/confscreen.cpp
+++ b/src/confscreen.cpp
@@ -448,7 +448,7 @@ bool TextWindow::EditControlDoneForConfiguration(const std::string &s) {
             Expr *e = Expr::From(s, /*popUpError=*/true);
             if(e) {
                 double ev = e->Eval();
-                if(fabs(ev) < 0.001 || isnan(ev)) {
+                if(fabs(ev) < 0.001 || IsReasonable(ev)) {
                     Error(_("Export scale must not be zero!"));
                 } else {
                     SS.exportScale = (float)ev;
@@ -460,7 +460,7 @@ bool TextWindow::EditControlDoneForConfiguration(const std::string &s) {
             Expr *e = Expr::From(s, /*popUpError=*/true);
             if(e) {
                 double ev = SS.ExprToMm(e);
-                if(isnan(ev) || ev < 0) {
+                if(IsReasonable(ev) || ev < 0) {
                     Error(_("Cutter radius offset must not be negative!"));
                 } else {
                     SS.exportOffset = (float)ev;
diff --git a/src/modify.cpp b/src/modify.cpp
index 48c10957..94b24787 100644
--- a/src/modify.cpp
+++ b/src/modify.cpp
@@ -383,7 +383,7 @@ void GraphicsWindow::MakeTangentArc() {
     if(fabs(tp[0] - t[0]) > 1e-3 || fabs(tp[1] - t[1]) > 1e-3 ||
         t[0] < 0.01 || t[1] < 0.01 ||
         t[0] > 0.99 || t[1] > 0.99 ||
-        isnan(t[0]) || isnan(t[1]))
+        IsReasonable(t[0]) || IsReasonable(t[1]))
     {
         Error(_("Couldn't round this corner. Try a smaller radius, or try "
                 "creating the desired geometry by hand with tangency "
diff --git a/src/solvespace.h b/src/solvespace.h
index 3a535a5a..165fd086 100644
--- a/src/solvespace.h
+++ b/src/solvespace.h
@@ -76,10 +76,6 @@ typedef struct _cairo_surface cairo_surface_t;
     dbp("tri: (%.3f %.3f %.3f) (%.3f %.3f %.3f) (%.3f %.3f %.3f)", \
         CO((tri).a), CO((tri).b), CO((tri).c))
 
-#ifndef isnan
-#   define isnan(x) (((x) != (x)) || (x > 1e11) || (x < -1e11))
-#endif
-
 namespace SolveSpace {
 
 using std::min;
@@ -98,6 +94,10 @@ __attribute__((__format__ (__printf__, 1, 2)))
 #endif
 std::string ssprintf(const char *fmt, ...);
 
+inline bool IsReasonable(double x) {
+    return std::isnan(x) || x > 1e11 || x < -1e11;
+}
+
 inline int WRAP(int v, int n) {
     // Clamp it to the range [0, n)
     while(v >= n) v -= n;
diff --git a/src/srf/ratpoly.cpp b/src/srf/ratpoly.cpp
index 9e70192a..1ffa3385 100644
--- a/src/srf/ratpoly.cpp
+++ b/src/srf/ratpoly.cpp
@@ -443,7 +443,7 @@ void SSurface::ClosestPointTo(Vector p, double *u, double *v, bool mustConverge)
         dbp("want %.3f %.3f %.3f", CO(p));
         dbp("distance = %g", (p.Minus(p0)).Magnitude());
     }
-    if(isnan(*u) || isnan(*v)) {
+    if(IsReasonable(*u) || IsReasonable(*v)) {
         *u = *v = 0;
     }
 }
diff --git a/src/system.cpp b/src/system.cpp
index 9c64b308..933f153a 100644
--- a/src/system.cpp
+++ b/src/system.cpp
@@ -293,7 +293,7 @@ bool System::NewtonSolve(int tag) {
         for(i = 0; i < mat.n; i++) {
             Param *p = param.FindById(mat.param[i]);
             p->val -= mat.X[i];
-            if(isnan(p->val)) {
+            if(IsReasonable(p->val)) {
                 // Very bad, and clearly not convergent
                 return false;
             }
@@ -306,7 +306,7 @@ bool System::NewtonSolve(int tag) {
         // Check for convergence
         converged = true;
         for(i = 0; i < mat.m; i++) {
-            if(isnan(mat.B.num[i])) {
+            if(IsReasonable(mat.B.num[i])) {
                 return false;
             }
             if(fabs(mat.B.num[i]) > CONVERGE_TOLERANCE) {
@@ -493,7 +493,7 @@ didnt_converge:
     SK.constraint.ClearTags();
     // Not using range-for here because index is used in additional ways
     for(i = 0; i < eq.n; i++) {
-        if(fabs(mat.B.num[i]) > CONVERGE_TOLERANCE || isnan(mat.B.num[i])) {
+        if(fabs(mat.B.num[i]) > CONVERGE_TOLERANCE || IsReasonable(mat.B.num[i])) {
             // This constraint is unsatisfied.
             if(!mat.eq[i].isFromConstraint()) continue;