diff --git a/scripts/bump_version.sh b/scripts/bump_version.sh
index f5d7033..7fc8c6c 100755
--- a/scripts/bump_version.sh
+++ b/scripts/bump_version.sh
@@ -96,7 +96,7 @@
   mv tmp library/CMakeLists.txt
 
   [ $VERBOSE ] && echo "Bumping SOVERSION for libmbedcrypto in library/Makefile"
-  sed -e "s/SOEXT_CRYPTO=so.[0-9]\{1,\}/SOEXT_CRYPTO=so.$SO_CRYPTO/g" < library/Makefile > tmp
+  sed -e "s/SOEXT_CRYPTO?=so.[0-9]\{1,\}/SOEXT_CRYPTO?=so.$SO_CRYPTO/g" < library/Makefile > tmp
   mv tmp library/Makefile
 fi
 
@@ -107,7 +107,7 @@
   mv tmp library/CMakeLists.txt
 
   [ $VERBOSE ] && echo "Bumping SOVERSION for libmbedx509 in library/Makefile"
-  sed -e "s/SOEXT_X509=so.[0-9]\{1,\}/SOEXT_X509=so.$SO_X509/g" < library/Makefile > tmp
+  sed -e "s/SOEXT_X509?=so.[0-9]\{1,\}/SOEXT_X509?=so.$SO_X509/g" < library/Makefile > tmp
   mv tmp library/Makefile
 fi
 
@@ -118,7 +118,7 @@
   mv tmp library/CMakeLists.txt
 
   [ $VERBOSE ] && echo "Bumping SOVERSION for libmbedtls in library/Makefile"
-  sed -e "s/SOEXT_TLS=so.[0-9]\{1,\}/SOEXT_TLS=so.$SO_TLS/g" < library/Makefile > tmp
+  sed -e "s/SOEXT_TLS?=so.[0-9]\{1,\}/SOEXT_TLS?=so.$SO_TLS/g" < library/Makefile > tmp
   mv tmp library/Makefile
 fi