diff --git a/configure.ac b/configure.ac
index 773731e0..54b54371 100644
--- a/configure.ac
+++ b/configure.ac
@@ -107,9 +107,9 @@ AC_ARG_WITH([linux-dir],
 
 AC_MSG_CHECKING([for Linux kernel sources])
 
-if test \! -r ${sourcedir}/.config; then
+if test \! -r ${sourcedir}/Makefile; then
     echo
-    AC_MSG_ERROR([No configured Linux kernel sources in $sourcedir])
+    AC_MSG_ERROR([No Linux kernel sources in $sourcedir])
 fi
 
 # Try to get kernel release string
@@ -127,7 +127,7 @@ fi
 
 if test -z "$kernelrelease"; then
     echo
-    AC_MSG_ERROR([Failed to extract Linux kernel version!])
+    AC_MSG_ERROR([Failed to extract Linux kernel version! Does ${sourcedir} contain prepared kernel sources?])
 fi
 
 if test ${kernelrelease%%.*} -gt 2; then