diff --git a/ANNOUNCE b/ANNOUNCE
index 83650d5b2..9db9dbfee 100644
--- a/ANNOUNCE
+++ b/ANNOUNCE
@@ -49,6 +49,13 @@ Version 1.6.19beta02 [August 17, 2015]
   Fixed some bad links in the man page.
   Changed "n bit" to "n-bit" in comments.
   Changed 0xnnnn constants to 0xnnnnUL.
+  Added signed/unsigned 16-bit safety net. This removes the dubious
+    0x8000 flag definitions on 16-bit systems. They aren't supported
+    yet the defs *probably* work, however it seems much safer to do this
+    and be advised if anyone, contrary to advice, is building libpng 1.6
+    on a 16-bit system. It also adds back various switch default clauses
+    for GCC; GCC errors out if they are not present (with an appropriately
+    high level of warnings).
 
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diff --git a/CHANGES b/CHANGES
index ca44ba264..7593cac3c 100644
--- a/CHANGES
+++ b/CHANGES
@@ -5328,6 +5328,13 @@ Version 1.6.19beta02 [August 17, 2015]
   Fixed some bad links in the man page.
   Changed "n bit" to "n-bit" in comments.
   Changed 0xnnnn constants to 0xnnnnUL.
+  Added signed/unsigned 16-bit safety net. This removes the dubious
+    0x8000 flag definitions on 16-bit systems. They aren't supported
+    yet the defs *probably* work, however it seems much safer to do this
+    and be advised if anyone, contrary to advice, is building libpng 1.6
+    on a 16-bit system. It also adds back various switch default clauses
+    for GCC; GCC errors out if they are not present (with an appropriately
+    high level of warnings).
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit
diff --git a/png.h b/png.h
index 8c2004e1e..106db7340 100644
--- a/png.h
+++ b/png.h
@@ -895,7 +895,9 @@ typedef png_unknown_chunk * * png_unknown_chunkpp;
 #define PNG_INFO_iCCP 0x1000   /* ESR, 1.0.6 */
 #define PNG_INFO_sPLT 0x2000   /* ESR, 1.0.6 */
 #define PNG_INFO_sCAL 0x4000   /* ESR, 1.0.6 */
+#if INT_MAX >= 0x8000 /* else this might break */
 #define PNG_INFO_IDAT 0x8000   /* ESR, 1.0.6 */
+#endif
 
 /* This is used for the transformation routines, as some of them
  * change these values for the row.  It also should enable using
@@ -999,7 +1001,9 @@ PNG_FUNCTION(void, (PNGCAPI *png_longjmp_ptr), PNGARG((jmp_buf, int)), typedef);
 #define PNG_TRANSFORM_GRAY_TO_RGB   0x2000      /* read only */
 /* Added to libpng-1.5.4 */
 #define PNG_TRANSFORM_EXPAND_16     0x4000      /* read only */
+#if INT_MAX >= 0x8000 /* else this might break */
 #define PNG_TRANSFORM_SCALE_16      0x8000      /* read only */
+#endif
 
 /* Flags for MNG supported features */
 #define PNG_FLAG_MNG_EMPTY_PLTE     0x01
diff --git a/pngread.c b/pngread.c
index 22054e1c9..1366140e7 100644
--- a/pngread.c
+++ b/pngread.c
@@ -1683,7 +1683,7 @@ decode_gamma(png_image_read_control *display, png_uint_32 value, int encoding)
          value *= 257;
          break;
 
-#ifdef PNG_DEBUG
+#ifdef __GNUC__
       default:
          png_error(display->image->opaque->png_ptr,
             "unexpected encoding (internal error)");
@@ -2858,7 +2858,7 @@ png_image_read_colormap(png_voidp argument)
             png_set_scale_16(png_ptr);
          break;
 
-#ifdef PNG_DEBUG
+#ifdef __GNUC__
       default:
          png_error(png_ptr, "bad data option (internal error)");
 #endif
@@ -3630,7 +3630,7 @@ png_image_read_background(png_voidp argument)
          }
          break;
 
-#ifdef PNG_DEBUG
+#ifdef __GNUC__
       default:
          png_error(png_ptr, "unexpected bit depth");
 #endif