diff --git a/libpng-manual.txt b/libpng-manual.txt
index 1acb70f92..b1fffdd0f 100644
--- a/libpng-manual.txt
+++ b/libpng-manual.txt
@@ -5140,10 +5140,6 @@ It can be set for testing debug or beta builds to ensure that they will compile
 when the build type is switched to RC or STABLE. In essence this overrides the
 PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable.
 
-Starting with libpng-1.6.19, it is an error to call png_set_sig_bytes()
-with a negative number.  Previously, a negative value was silently converted
-to zero.
-
 XIII.  Detecting libpng
 
 The png_get_io_ptr() function has been present since libpng-0.88, has never
diff --git a/libpng.3 b/libpng.3
index af68c5984..2e2f12622 100644
--- a/libpng.3
+++ b/libpng.3
@@ -5648,10 +5648,6 @@ It can be set for testing debug or beta builds to ensure that they will compile
 when the build type is switched to RC or STABLE. In essence this overrides the
 PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable.
 
-Starting with libpng-1.6.19, it is an error to call png_set_sig_bytes()
-with a negative number.  Previously, a negative value was silently converted
-to zero.
-
 .SH XIII.  Detecting libpng
 
 The png_get_io_ptr() function has been present since libpng-0.88, has never
diff --git a/png.c b/png.c
index c57b071a9..541727760 100644
--- a/png.c
+++ b/png.c
@@ -33,6 +33,9 @@ png_set_sig_bytes(png_structrp png_ptr, int num_bytes)
    if (png_ptr == NULL)
       return;
 
+   if (num_bytes < 0)
+      nb = 0;
+
    if (nb > 8)
       png_error(png_ptr, "Too many bytes for PNG signature");