diff --git a/tools/buildman/README b/tools/buildman/README
index d20508f..68465b4 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -670,7 +670,9 @@
 This will use 'make ENABLE_AT91_TEST=1 BUILD_TAG=442' for snapper9260
 and 'make ENABLE_AT91_TEST=1 BUILD_TAG=443' for snapper9g45. A special
 variable ${target} is available to access the target name (snapper9260 and
-snapper9g20 in this case). Variables are resolved recursively.
+snapper9g20 in this case). Variables are resolved recursively. Note that
+variables can only contain the characters A-Z, a-z, 0-9, hyphen (-) and
+underscore (_).
 
 It is expected that any variables added are dealt with in U-Boot's
 config.mk file and documented in the README.
diff --git a/tools/buildman/toolchain.py b/tools/buildman/toolchain.py
index 1b9771f..0e91294 100644
--- a/tools/buildman/toolchain.py
+++ b/tools/buildman/toolchain.py
@@ -198,7 +198,7 @@
         >>> tcs.ResolveReferences(var_dict, 'this=${oblique}_set${first}nd')
         'this=OBLIQUE_setfi2ndrstnd'
         """
-        re_var = re.compile('(\$\{[a-z0-9A-Z]{1,}\})')
+        re_var = re.compile('(\$\{[-_a-z0-9A-Z]{1,}\})')
 
         while True:
             m = re_var.search(args)