printf("Combining two %dx%d matrices\n",mat1[k].nx,mat1[k].ny);
rmat1 = matrix2real(&mat1[k], NULL);
rmat2 = matrix2real(&mat2[k], NULL);
+ if (NULL == rmat1 || NULL == rmat2)
+ {
+ gmx_fatal(FARGS, "Could not extract real data from %s xpm matrices. Note that, e.g.,\n"
+ "g_rms and g_mdmat provide such data, but not do_dssp.\n",
+ (NULL==rmat1 && NULL==rmat2) ? "both" : "one of the" );
+ }
rlo=1e38;
rhi=-1e38;
for(j=0; j<mat1[k].ny; j++)