diff --git a/libvips/deprecated/format.c b/libvips/deprecated/format.c
index 03ea8fd1..fd94a2f0 100644
--- a/libvips/deprecated/format.c
+++ b/libvips/deprecated/format.c
@@ -176,7 +176,7 @@ im_isvips( const char *filename )
 {
 	unsigned char buf[4];
 
-	if( im__get_bytes( filename, buf, 4 ) ) {
+	if( im__get_bytes( filename, buf, 4 ) == 4 ) {
 		if( buf[0] == 0x08 && buf[1] == 0xf2 &&
 			buf[2] == 0xa6 && buf[3] == 0xb6 )
 			/* SPARC-order VIPS image.
@@ -212,7 +212,7 @@ vips_flags( const char *filename )
 
 	flags = VIPS_FORMAT_PARTIAL;
 
-	if( im__get_bytes( filename, buf, 4 ) &&
+	if( im__get_bytes( filename, buf, 4 ) == 4 &&
 		buf[0] == 0x08 && 
 		buf[1] == 0xf2 &&
 		buf[2] == 0xa6 && 
diff --git a/libvips/foreign/magick7load.c b/libvips/foreign/magick7load.c
index d729db9b..a81a56af 100644
--- a/libvips/foreign/magick7load.c
+++ b/libvips/foreign/magick7load.c
@@ -751,15 +751,13 @@ G_DEFINE_TYPE( VipsForeignLoadMagick7File, vips_foreign_load_magick7_file,
 static gboolean
 ismagick7( const char *filename )
 {
-	/* Fetch the first 100 bytes. Hopefully that'll be enough.
+	/* Fetch up to the first 100 bytes. Hopefully that'll be enough.
 	 */
 	unsigned char buf[100];
+	int len;
 
-	/* Files shorter than 100 bytes will leave nonsense at the end of buf,
-	 * but it shouldn't matter.
-	 */
-	return( vips__get_bytes( filename, buf, 100 ) &&
-		magick_ismagick( buf, 100 ) );
+	return( (len = vips__get_bytes( filename, buf, 100 )) > 10 &&
+		magick_ismagick( buf, len ) );
 }
 
 static int
diff --git a/libvips/foreign/magickload.c b/libvips/foreign/magickload.c
index a1215a30..bfb26566 100644
--- a/libvips/foreign/magickload.c
+++ b/libvips/foreign/magickload.c
@@ -173,15 +173,13 @@ G_DEFINE_TYPE( VipsForeignLoadMagickFile, vips_foreign_load_magick_file,
 static gboolean
 ismagick( const char *filename )
 {
-	/* Fetch the first 100 bytes. Hopefully that'll be enough.
+	/* Fetch up to the first 100 bytes. Hopefully that'll be enough.
 	 */
 	unsigned char buf[100];
+	int len;
 
-	/* Files shorter than 100 bytes will leave nonsense at the end of buf,
-	 * but it shouldn't matter.
-	 */
-	return( vips__get_bytes( filename, buf, 100 ) &&
-		magick_ismagick( buf, 100 ) );
+	return( (len = vips__get_bytes( filename, buf, 100 )) > 10 &&
+		magick_ismagick( buf, len ) );
 }
 
 /* Unfortunately, libMagick does not support header-only reads very well. See
diff --git a/libvips/include/vips/util.h b/libvips/include/vips/util.h
index fe0d4b7e..d9cdf93e 100644
--- a/libvips/include/vips/util.h
+++ b/libvips/include/vips/util.h
@@ -266,8 +266,8 @@ char *vips__file_read( FILE *fp, const char *name, size_t *length_out );
 char *vips__file_read_name( const char *name, const char *fallback_dir, 
 	size_t *length_out );
 int vips__file_write( void *data, size_t size, size_t nmemb, FILE *stream );
-guint64 vips__get_bytes( const char *filename, 
-	unsigned char buf[], guint64 len );
+gint64 vips__get_bytes( const char *filename, 
+	unsigned char buf[], gint64 len );
 int vips__fgetc( FILE *fp );
 
 GValue *vips__gvalue_ref_string_new( const char *text );
diff --git a/libvips/iofuncs/util.c b/libvips/iofuncs/util.c
index 2399485d..99c67641 100644
--- a/libvips/iofuncs/util.c
+++ b/libvips/iofuncs/util.c
@@ -874,13 +874,13 @@ vips__file_write( void *data, size_t size, size_t nmemb, FILE *stream )
  * types, so we must read binary. 
  *
  * Return the number of bytes actually read (the file might be shorter than
- * len), or 0 for error.
+ * len), or -1 for error.
  */
-guint64
-vips__get_bytes( const char *filename, unsigned char buf[], guint64 len )
+gint64
+vips__get_bytes( const char *filename, unsigned char buf[], gint64 len )
 {
 	int fd;
-	guint64 bytes_read;
+	gint64 bytes_read;
 
 	/* File may not even exist (for tmp images for example!)
 	 * so no hasty messages. And the file might be truncated, so no error