diff --git a/base/applications/calc/winmain.c b/base/applications/calc/winmain.c
index 974216b6f14..b2ab16cd0e2 100644
--- a/base/applications/calc/winmain.c
+++ b/base/applications/calc/winmain.c
@@ -1635,7 +1635,7 @@ static INT_PTR CALLBACK DlgMainProc(HWND hWnd, UINT msg, WPARAM wp, LPARAM lp)
         case IDC_BUTTON_D:
         case IDC_BUTTON_E:
         case IDC_BUTTON_F:
-            calc.is_nan = FALSE;
+            if (calc.is_nan) break;
             build_operand(hWnd, LOWORD(wp));
             return TRUE;
         case IDC_BUTTON_PERCENT:
@@ -1701,6 +1701,7 @@ static INT_PTR CALLBACK DlgMainProc(HWND hWnd, UINT msg, WPARAM wp, LPARAM lp)
             }
             return TRUE;
         case IDC_BUTTON_BACK:
+            if (calc.is_nan) break;
             if (calc.sci_in) {
                 if (calc.esp == 0) {
                     TCHAR *ptr;