diff --git a/src/libexpr/print.cc b/src/libexpr/print.cc
index 1ff026b3db3..cdc9f6dbeec 100644
--- a/src/libexpr/print.cc
+++ b/src/libexpr/print.cc
@@ -176,6 +176,7 @@ class Printer
     void decreaseIndent()
     {
         if (options.prettyPrint()) {
+            assert(indent.size() >= options.prettyIndent);
             indent.resize(indent.size() - options.prettyIndent);
         }
     }