diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
index 41d4b17e41..eb2b475ca4 100644
--- a/fpu/softfloat-parts.c.inc
+++ b/fpu/softfloat-parts.c.inc
@@ -354,7 +354,7 @@ static FloatPartsN *partsN(addsub)(FloatPartsN *a, FloatPartsN *b,
                 return a;
             }
             /* Inf - Inf */
-            float_raise(float_flag_invalid, s);
+            float_raise(float_flag_invalid | float_flag_invalid_isi, s);
             parts_default_nan(a, s);
             return a;
         }
@@ -494,6 +494,7 @@ static FloatPartsN *partsN(muladd)(FloatPartsN *a, FloatPartsN *b,
 
         if (ab_mask & float_cmask_inf) {
             if (c->cls == float_class_inf && a->sign != c->sign) {
+                float_raise(float_flag_invalid | float_flag_invalid_isi, s);
                 goto d_nan;
             }
             goto return_inf;
diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
index 65a43aff59..eaa12e1e00 100644
--- a/include/fpu/softfloat-types.h
+++ b/include/fpu/softfloat-types.h
@@ -152,6 +152,7 @@ enum {
     float_flag_inexact         = 0x0010,
     float_flag_input_denormal  = 0x0020,
     float_flag_output_denormal = 0x0040,
+    float_flag_invalid_isi     = 0x0080,  /* inf - inf */
 };
 
 /*