diff --git a/terminal.c b/terminal.c
index f4902fef..d2ca8a48 100644
--- a/terminal.c
+++ b/terminal.c
@@ -1045,6 +1045,27 @@ static int sblines(Terminal *term)
     return sblines;
 }
 
+static void null_line_error(Terminal *term, int y, int lineno,
+                            tree234 *whichtree, int treeindex,
+                            const char *varname)
+{
+    extern const char commitid[]; /* in version.c */
+    modalfatalbox("%s==NULL in terminal.c\n"
+                  "lineno=%d y=%d w=%d h=%d\n"
+                  "count(scrollback=%p)=%d\n"
+                  "count(screen=%p)=%d\n"
+                  "count(alt=%p)=%d alt_sblines=%d\n"
+                  "whichtree=%p treeindex=%d\n"
+                  "commitid=%s\n\n"
+                  "Please contact <putty@projects.tartarus.org> "
+                  "and pass on the above information.",
+                  varname, lineno, y, term->cols, term->rows,
+                  term->scrollback, count234(term->scrollback),
+                  term->screen, count234(term->screen),
+                  term->alt_screen, count234(term->alt_screen),
+                  term->alt_sblines, whichtree, treeindex, commitid);
+}
+
 /*
  * Retrieve a line of the screen or of the scrollback, according to
  * whether the y coordinate is non-negative or negative
@@ -1079,29 +1100,16 @@ static termline *lineptr(Terminal *term, int y, int lineno, int screen)
     }
     if (whichtree == term->scrollback) {
 	unsigned char *cline = index234(whichtree, treeindex);
+        if (!cline)
+            null_line_error(term, y, lineno, whichtree, treeindex, "cline");
 	line = decompressline(cline, NULL);
     } else {
 	line = index234(whichtree, treeindex);
     }
 
     /* We assume that we don't screw up and retrieve something out of range. */
-    if (line == NULL) {
-        extern const char commitid[]; /* in version.c */
-	modalfatalbox("line==NULL in terminal.c\n"
-                      "lineno=%d y=%d w=%d h=%d\n"
-                      "count(scrollback=%p)=%d\n"
-                      "count(screen=%p)=%d\n"
-                      "count(alt=%p)=%d alt_sblines=%d\n"
-                      "whichtree=%p treeindex=%d\n"
-                      "commitid=%s\n\n"
-                      "Please contact <putty@projects.tartarus.org> "
-                      "and pass on the above information.",
-                      lineno, y, term->cols, term->rows,
-                      term->scrollback, count234(term->scrollback),
-                      term->screen, count234(term->screen),
-                      term->alt_screen, count234(term->alt_screen),
-                      term->alt_sblines, whichtree, treeindex, commitid);
-    }
+    if (line == NULL)
+        null_line_error(term, y, lineno, whichtree, treeindex, "line");
     assert(line != NULL);
 
     /*