diff --git a/common.c b/common.c
index a63cbce91..cef60d0de 100644
--- a/common.c
+++ b/common.c
@@ -548,14 +548,17 @@ int read_blocked(int fd, void *buf, size_t count)
 void debug( int level, const wchar_t *msg, ... )
 {
 	va_list va;
+
 	string_buffer_t sb;
 	string_buffer_t sb2;
+
+	int errno_old = errno;
 	
-	CHECK( msg, );
-		
 	if( level > debug_level )
 		return;
 
+	CHECK( msg, );
+		
 	sb_init( &sb );
 	sb_init( &sb2 );
 
@@ -570,6 +573,8 @@ void debug( int level, const wchar_t *msg, ... )
 
 	sb_destroy( &sb );	
 	sb_destroy( &sb2 );	
+
+	errno = errno_old;
 }
 
 void write_screen( const wchar_t *msg, string_buffer_t *buff )
diff --git a/common.h b/common.h
index 323dd1cae..d6559cd0a 100644
--- a/common.h
+++ b/common.h
@@ -305,6 +305,10 @@ int read_blocked(int fd, void *buf, size_t count);
    Issue a debug message with printf-style string formating and
    automatic line breaking. The string will begin with the string \c
    program_name, followed by a colon and a whitespace.
+
+   Because debug is often called to tell the user about an error,
+   before using wperror to give a specific error message, debug will
+   never ever modify the value of errno.
    
    \param level the priority of the message. Lower number means higher priority. Messages with a priority_number higher than \c debug_level will be ignored..
    \param msg the message format string.