diff --git a/initialize/env.go b/initialize/env.go
index 4161898..2019c3e 100644
--- a/initialize/env.go
+++ b/initialize/env.go
@@ -19,6 +19,9 @@ type Environment struct {
 
 // TODO(jonboulle): this is getting unwieldy, should be able to simplify the interface somehow
 func NewEnvironment(root, configRoot, workspace, netconfType, sshKeyName string, substitutions map[string]string) *Environment {
+	if substitutions == nil {
+		substitutions = make(map[string]string)
+	}
 	// If certain values are not in the supplied substitution, fall back to retrieving them from the environment
 	for k, v := range map[string]string{
 		"$public_ipv4":  os.Getenv("COREOS_PUBLIC_IPV4"),
diff --git a/initialize/env_test.go b/initialize/env_test.go
index 756c957..b48ae21 100644
--- a/initialize/env_test.go
+++ b/initialize/env_test.go
@@ -41,6 +41,12 @@ ExecStop=/usr/bin/echo $unknown`,
 			"$private_ipv4\n$public_ipv4",
 			"5.6.7.8\n1.2.3.4",
 		},
+		{
+			// No substitutions
+			nil,
+			"$private_ipv4\nfoobar",
+			"5.6.7.8\nfoobar",
+		},
 	} {
 
 		env := NewEnvironment("./", "./", "./", "", "", tt.subs)