diff --git a/var/spack/repos/builtin/packages/pgplot/package.py b/var/spack/repos/builtin/packages/pgplot/package.py
index 9664ea652ba..6b867dc9ffa 100644
--- a/var/spack/repos/builtin/packages/pgplot/package.py
+++ b/var/spack/repos/builtin/packages/pgplot/package.py
@@ -57,6 +57,7 @@ def edit(self, spec, prefix):
         if '+png' in spec:
             libs += ' ' + self.spec['libpng'].libs.ld_flags
 
+        sub = {}
         if spec.satisfies('%gcc'):
             fib = " -fallow-invalid-boz" if spec.satisfies('%gcc@10:') else ""