diff -urN netpbm-10.51.00-orig/converter/other/pnmtopng.c netpbm-10.51.00/converter/other/pnmtopng.c
--- netpbm-10.51.00-orig/converter/other/pnmtopng.c	2009-12-10 01:25:43 +0900
+++ netpbm-10.51.00/converter/other/pnmtopng.c	2011-01-05 19:25:44 +0900
@@ -1981,12 +1981,9 @@
                     unsigned int paletteSize;
                     unsigned int transSize;
                     if (alpha)
-                        tryAlphaPalette(ifP, cols, rows, maxval, format,
-                                        rasterPos, alpha_mask, alphaMaxval,
-                                        pfP,
-                                        palette_pnm, &paletteSize, 
-                                        trans_pnm, &transSize,
-                                        noColormapReasonP);
+                        asprintfN(noColormapReasonP, 
+                                  "alpha values are not permitted in PNG "
+                                  "palettes");
 
                     else {
                         *noColormapReasonP = NULL;