Index: src/expr.c
==================================================================
--- src/expr.c
+++ src/expr.c
@@ -3643,11 +3643,20 @@
           pCol = pTab->aCol + iCol;
           testcase( iCol!=sqlite3TableColumnToStorage(pTab,iCol) );
           iSrc = sqlite3TableColumnToStorage(pTab, iCol) - pParse->iSelfTab;
 #ifndef SQLITE_OMIT_GENERATED_COLUMNS
           if( pCol->colFlags & COLFLAG_GENERATED ){
-            sqlite3ExprCodeGeneratedColumn(pParse, pCol, iSrc);
+            if( pCol->colFlags & COLFLAG_BUSY ){
+              sqlite3ErrorMsg(pParse, "generated column loop on \"%s\"",
+                              pCol->zName);
+              return 0;
+            }
+            pCol->colFlags |= COLFLAG_BUSY;
+            if( pCol->colFlags & COLFLAG_NOTAVAIL ){
+              sqlite3ExprCodeGeneratedColumn(pParse, pCol, iSrc);
+            }
+            pCol->colFlags &= ~(COLFLAG_BUSY|COLFLAG_NOTAVAIL);
             return iSrc;
           }else
 #endif /* SQLITE_OMIT_GENERATED_COLUMNS */
           if( pCol->affinity==SQLITE_AFF_REAL ){
             sqlite3VdbeAddOp2(v, OP_SCopy, iSrc, target);

Index: test/gencol1.test
==================================================================
--- test/gencol1.test
+++ test/gencol1.test
@@ -231,7 +231,17 @@
     c2 CHECK (1 = c1)
   );
   INSERT INTO t0 VALUES (0),(99);
   SELECT * FROM t0;
 } {1 1 0 1 1 99}
+do_catchsql_test gencol1-8.20 {
+  DROP TABLE IF EXISTS t0;
+  CREATE TABLE t0(
+    c0,
+    c1 AS(c0 + c2),
+    c2 AS(c1) CHECK(c2)
+  );
+  UPDATE t0 SET c0 = NULL;
+} {1 {generated column loop on "c2"}}
+
 
 finish_test