diff --git a/lisp/ox-beamer.el b/lisp/ox-beamer.el
index 5a87effc6..921bab6eb 100644
--- a/lisp/ox-beamer.el
+++ b/lisp/ox-beamer.el
@@ -828,7 +828,8 @@ holding export options."
 		       (replace-regexp-in-string
 			"^[ \t]*\\\\documentclass\\(\\(\\[.*\\]\\)?\\)"
 			class-options header t nil 1)))))
-	  (when document-class-string
+	  (if (not document-class-string)
+	      (user-error "Unknown LaTeX class `%s'")
 	    (org-latex-guess-babel-language
 	     (org-latex-guess-inputenc
 	      (org-splice-latex-header
diff --git a/lisp/ox-latex.el b/lisp/ox-latex.el
index d8a2af431..a49be0c37 100644
--- a/lisp/ox-latex.el
+++ b/lisp/ox-latex.el
@@ -1108,7 +1108,8 @@ holding export options."
 		       (replace-regexp-in-string
 			"^[ \t]*\\\\documentclass\\(\\(\\[.*\\]\\)?\\)"
 			class-options header t nil 1)))))
-	  (when document-class-string
+	  (if (not document-class-string)
+	      (user-error "Unknown LaTeX class `%s'")
 	    (org-latex-guess-babel-language
 	     (org-latex-guess-inputenc
 	      (org-splice-latex-header