diff --git a/de.prob.core/META-INF/MANIFEST.MF b/de.prob.core/META-INF/MANIFEST.MF
index 3e4e524df1871574909e1a0b764af1489cd07c8d..2a48294611e3b745cac6e2e7d0e9467a7f823650 100644
--- a/de.prob.core/META-INF/MANIFEST.MF
+++ b/de.prob.core/META-INF/MANIFEST.MF
@@ -128,4 +128,4 @@ Bundle-ClassPath: .,
  lib/dependencies/xmlpull-1.1.3.1.jar,
  lib/dependencies/xpp3_min-1.1.4c.jar,
  lib/dependencies/xstream-1.4.3.jar
-Import-Package: org.eventb.theory.core
+Import-Package: org.eventb.theory.core;resolution:=optional