diff --git a/src/envmon.sh b/src/envmon.sh
index 40c2f1e..770f376 100644
--- a/src/envmon.sh
+++ b/src/envmon.sh
@@ -70,6 +70,21 @@ __pkgsh-envmon-envload()
 }
 
 
+##
+# Exits with a zero status if $name exists in the environment cache and is
+# not empty
+#
+# Otherwise, exits with a non-zero status.
+#
+_pkgsh-envmon-is-cached()
+{
+  local -r name="$1"
+
+  # FIXME: this will fail if the in-memory value is empty
+  [ -n "${__pkgsh_envmon_env[$var]}" ]
+}
+
+
 ##
 # Output null-byte-delimited names and values of variables added to the
 # environment since the last diff and update the environment cache
@@ -87,12 +102,12 @@ __pkgsh-envmon-dodiff-adds()
 {
   local -r var="$1" val="$2"
 
-  # ignore if it already exists (FIXME: this will fail if the in-memory
-  # value is empty)
-  [ -z "${__pkgsh_envmon_env[$var]}" ] || return 0
+  if _pkgsh-envmon-is-cached "$var"; then
+    return 0
+  fi
 
   # output the difference and immediately add to the environment in memory
   echo -ne "$var=$val\000"
-  __pkgsh_envmon_env[$var]="$val"
+  __pkgsh-envmon-envload "$var" "$val"
 }