diff --git a/Makefile.am b/Makefile.am
index 291565e..5b343f5 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -4,6 +4,6 @@ pkgconfig_DATA = cmph.pc
 if USE_CXXMPH
   pkgconfig_DATA += cxxmph.pc
 endif
-ACLOCAL_AMFLAGS=-I m4
+ACLOCAL_AMFLAGS="-Im4"
 
 pkgconfigdir = $(libdir)/pkgconfig