diff --git a/confutils.nim b/confutils.nim
index 16ae972..da95dd7 100644
--- a/confutils.nim
+++ b/confutils.nim
@@ -129,7 +129,7 @@ when defined(nimscript):
     100000
 
 else:
-  template appInvocation: string =
+  proc appInvocation: string =
     try:
       getAppFilename().splitFile.name
     except OSError:
@@ -1043,7 +1043,12 @@ when hasSerialization:
 func constructEnvKey*(prefix: string, key: string): string {.raises: [].} =
   ## Generates env. variable names from keys and prefix following the
   ## IEEE Open Group env. variable spec: https://pubs.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap08.html
-  (prefix & "_" & key).toUpperAscii.multiReplace(("-", "_"), (" ", "_"))
+  let fullKey =
+    if prefix.len > 0:
+      prefix & "_" & key
+    else:
+      key
+  fullKey.toUpperAscii.multiReplace(("-", "_"), (" ", "_"))
 
 # On Posix there is no portable way to get the command
 # line from a DLL and thus the proc isn't defined in this environment.
@@ -1051,6 +1056,11 @@ func constructEnvKey*(prefix: string, key: string): string {.raises: [].} =
 when not declared(commandLineParams):
   proc commandLineParams(): seq[string] = discard
 
+proc defaultEnvVarsPrefix(): string =
+  result = appInvocation()
+  if result.len == 0:
+    result = "_"
+
 proc loadImpl[C, SecondarySources](
     Configuration: typedesc[C],
     cmdLine = commandLineParams(),
@@ -1063,7 +1073,7 @@ proc loadImpl[C, SecondarySources](
     secondarySources: proc (
         config: Configuration, sources: ref SecondarySources
     ) {.gcsafe, raises: [ConfigurationError].} = nil,
-    envVarsPrefix = appInvocation(),
+    envVarsPrefix = defaultEnvVarsPrefix(),
     termWidth = 0
 ): Configuration {.raises: [ConfigurationError].} =
   ## Loads a program configuration by parsing command-line arguments
@@ -1349,7 +1359,7 @@ template load*(
     quitOnFailure = true,
     ignoreUnknown = false,
     secondarySources: untyped = nil,
-    envVarsPrefix = appInvocation(),
+    envVarsPrefix = defaultEnvVarsPrefix(),
     termWidth = 0): untyped =
   block:
     let secondarySourcesRef = generateSecondarySources(Configuration)
diff --git a/tests/test_envvar.nim b/tests/test_envvar.nim
index a02d1bf..4f87b25 100644
--- a/tests/test_envvar.nim
+++ b/tests/test_envvar.nim
@@ -64,4 +64,9 @@ proc testEnvvar() =
       check conf.somObject.name.string == "helloObject"
       check conf.somObject.isNice.bool == true
 
+    test "env vars no prefix":
+      putEnv("DATA_DIR", "ENV VAR DATADIR")
+      let conf = TestConf.load(envVarsPrefix="")
+      check conf.dataDir.string == "ENV VAR DATADIR"
+
 testEnvvar()