diff --git a/academic/primer3/primer3.SlackBuild b/academic/primer3/primer3.SlackBuild
index d3ae1b399c..514f65fd23 100644
--- a/academic/primer3/primer3.SlackBuild
+++ b/academic/primer3/primer3.SlackBuild
@@ -75,13 +75,11 @@ patch -p1 -i $CWD/gcc-7.patch
 cd ./src
 
 # Use our CFLAGS
-sed -i "/^CFLAGS/s/=/+=/" Makefile
+sed -i -e "s|-O2|$SLKCFLAGS|" -e "s|-g -Wall|-fpermissive|" Makefile
 
 # Look for configuration files in /usr/share instead of /opt
 sed -i "s:/opt/primer3_config:/usr/share/$PRGNAM/primer3_config:g" thal_main.c primer3_boulder_main.c
 
-CFLAGS="$SLKCFLAGS" \
-CXXFLAGS="$SLKCFLAGS" \
 make
 
 # This is recommended, but the tests take a really long time. Be patient