Index: src/expr.c
==================================================================
--- src/expr.c
+++ src/expr.c
@@ -2865,10 +2865,11 @@
   if( addrOnce ){
     sqlite3VdbeJumpHere(v, addrOnce);
     /* Subroutine return */
     sqlite3VdbeAddOp1(v, OP_Return, pExpr->y.sub.regReturn);
     sqlite3VdbeChangeP1(v, pExpr->y.sub.iAddr-1, sqlite3VdbeCurrentAddr(v)-1);
+    sqlite3ClearTempRegCache(pParse);
   }
 }
 #endif /* SQLITE_OMIT_SUBQUERY */
 
 /*
@@ -2975,10 +2976,11 @@
     sqlite3VdbeJumpHere(v, addrOnce);
 
     /* Subroutine return */
     sqlite3VdbeAddOp1(v, OP_Return, pExpr->y.sub.regReturn);
     sqlite3VdbeChangeP1(v, pExpr->y.sub.iAddr-1, sqlite3VdbeCurrentAddr(v)-1);
+    sqlite3ClearTempRegCache(pParse);
   }
 
   return rReg;
 }
 #endif /* SQLITE_OMIT_SUBQUERY */
@@ -5606,10 +5608,15 @@
   }
 }
 
 /*
 ** Mark all temporary registers as being unavailable for reuse.
+**
+** Always invoke this procedure after coding a subroutine or co-routine
+** that might be invoked from other parts of the code, to ensure that
+** the sub/co-routine does not use registers in common with the code that
+** invokes the sub/co-routine.
 */
 void sqlite3ClearTempRegCache(Parse *pParse){
   pParse->nTempReg = 0;
   pParse->nRangeReg = 0;
 }

Index: test/tkt-3a77c9714e.test
==================================================================
--- test/tkt-3a77c9714e.test
+++ test/tkt-3a77c9714e.test
@@ -1,6 +1,6 @@
-# 2011 December 06
+# 2011-12-06
 #
 # The author disclaims copyright to this source code.  In place of
 # a legal notice, here is a blessing:
 #
 #    May you do good and not evil.
@@ -66,7 +66,21 @@
         WHERE Connected=SrcWord LIMIT 1
       )
     )
 } {FACTORING FACTOR SWIMMING SWIMM} 
 
+# Similar problem discovered by dbsqlfuzz on 2019-09-18
+#
+do_execsql_test 3.0 {
+  DROP TABLE IF EXISTS t1;
+  CREATE TABLE t1(i INT PRIMARY KEY, a, b);
+  INSERT INTO t1 VALUES(NULL,'one','i');
+  CREATE INDEX i1a ON t1(a);
+  CREATE INDEX i1b ON t1(b);
+  SELECT (SELECT 1
+            FROM (SELECT 1 FROM t1 WHERE a=1 OR b='i')
+           WHERE a='o'
+              OR b IN (SELECT a=('b' IN (SELECT 'a'))))
+    FROM t1;
+} {{}}
 
 finish_test