Adapted patch from https://github.com/sim82/papara_nt/issues/3

pvec.h.patch

--- pvec.h	2018-12-09 12:08:14.727171805 +0100
+++ pvec_mod.h	2018-12-09 12:09:36.186071505 +0100
@@ -539,22 +539,26 @@
 
 
     static inline double gap_posterior( double v1, double v2 ) {
-    	assert( pgap_model.is_valid_ptr() );
-    	//return v1 / (v1 + v2);
+        assert( pgap_model.is_valid_ptr() );
+        //return v1 / (v1 + v2);
 
-    	v1 *= 1 - pgap_model->gap_freq();
-    //	throw std::runtime_error( "i think there is an error in this function. why v1 in the next line?");
-    	v2 *= pgap_model->gap_freq();
-
-    	float v = float(v1 / (v1 + v2));
-
-    	if( v != v ) {
-     		std::cerr << "meeeeep: " << v1 << " " << v2 << "\n";
-
-     		throw std::runtime_error("bailing out.");
-    	}
-
-    	return v;
+        v1 *= 1 - pgap_model->gap_freq();
+        //	throw std::runtime_error( "i think there is an error in this function. why v1 in the next line?");
+        v2 *= pgap_model->gap_freq();
+
+        float v;
+
+        if (v1 != 0) {
+            v = float(v1 / (v1 + v2));
+            if( v != v ) {
+                std::cerr << "meeeeep: " << v1 << " " << v2 << "\n";
+
+                throw std::runtime_error("bailing out.");
+            }
+        } else {
+            v = v1;
+        }
+        return v;
     }
 
     static inline double gap_ancestral_probability( double v1, double v2 ) {