diff --git a/tools/geometryc/geometryc.cpp b/tools/geometryc/geometryc.cpp
index 23ecb108..f748e5c7 100644
--- a/tools/geometryc/geometryc.cpp
+++ b/tools/geometryc/geometryc.cpp
@@ -504,11 +504,14 @@ int main(int _argc, const char* _argv[])
 				Triangle triangle;
 				memset(&triangle, 0, sizeof(Triangle) );
 
+				const int numNormals   = (int)normals.size();
+				const int numTexcoords = (int)texcoords.size();
+				const int numPositions = (int)positions.size();
 				for (uint32_t edge = 0, numEdges = argc-1; edge < numEdges; ++edge)
 				{
 					Index3 index;
-					index.m_texcoord = -1;
-					index.m_normal = -1;
+					index.m_texcoord = 0;
+					index.m_normal = 0;
 					index.m_vertexIndex = -1;
 
 					char* vertex = argv[edge+1];
@@ -521,13 +524,16 @@ int main(int _argc, const char* _argv[])
 						if (NULL != normal)
 						{
 							*normal++ = '\0';
-							index.m_normal = atoi(normal)-1;
+							const int nn = atoi(normal);
+							index.m_normal = (nn < 0) ? nn+numNormals : nn-1;
 						}
 
-						index.m_texcoord = atoi(texcoord)-1;
+						const int tex = atoi(texcoord);
+						index.m_texcoord = (tex < 0) ? tex+numTexcoords : tex-1;
 					}
 
-					index.m_position = atoi(vertex)-1;
+					const int pos = atoi(vertex);
+					index.m_position = (pos < 0) ? pos+numPositions : pos-1;
 
 					uint64_t hash0 = index.m_position;
 					uint64_t hash1 = uint64_t(index.m_texcoord)<<20;
@@ -710,8 +716,8 @@ int main(int _argc, const char* _argv[])
 	bool hasTexcoord;
 	{
 		Index3Map::const_iterator it = indexMap.begin();
-		hasNormal = -1 != it->second.m_normal;
-		hasTexcoord = -1 != it->second.m_texcoord;
+		hasNormal   = 0 != it->second.m_normal;
+		hasTexcoord = 0 != it->second.m_texcoord;
 
 		if (!hasTexcoord
 		&&  texcoords.size() == positions.size() )