diff --git a/src/backend/modern.lisp b/src/backend/modern.lisp
index 4c15fcf..b9a8450 100644
--- a/src/backend/modern.lisp
+++ b/src/backend/modern.lisp
@@ -185,10 +185,10 @@ as a fallback when a keyword is not in *named-colors*.")
         ;; $LINES from the shell, which reflect the true size at
         ;; process start even when ioctl on stdout's fd disagrees.
         (or (ignore-errors
-              (let ((cols (parse-integer (sb-ext:posix-getenv "COLUMNS")
-                                         :junk-allowed t))
-                    (rows (parse-integer (sb-ext:posix-getenv "LINES")
-                                         :junk-allowed t)))
+              (let* ((cstr (sb-ext:posix-getenv "COLUMNS"))
+                     (rstr (sb-ext:posix-getenv "LINES"))
+                     (cols (when cstr (parse-integer cstr :junk-allowed t)))
+                     (rows (when rstr (parse-integer rstr :junk-allowed t))))
                 (when (and cols rows (> cols 0) (> rows 0))
                   (values cols rows))))
             (values w h)))))
diff --git a/src/backend/simple.lisp b/src/backend/simple.lisp
index 570ab9b..e2704a4 100644
--- a/src/backend/simple.lisp
+++ b/src/backend/simple.lisp
@@ -44,10 +44,10 @@
     (if (and (> w 80) (> h 24))
         (values w h)
         (or (ignore-errors
-              (let ((cols (parse-integer (sb-ext:posix-getenv "COLUMNS")
-                                         :junk-allowed t))
-                    (rows (parse-integer (sb-ext:posix-getenv "LINES")
-                                         :junk-allowed t)))
+              (let* ((cstr (sb-ext:posix-getenv "COLUMNS"))
+                     (rstr (sb-ext:posix-getenv "LINES"))
+                     (cols (when cstr (parse-integer cstr :junk-allowed t)))
+                     (rows (when rstr (parse-integer rstr :junk-allowed t))))
                 (when (and cols rows (> cols 0) (> rows 0))
                   (values cols rows))))
             (values w h)))))