diff --git a/src/main/scala/millfork/compiler/AbstractStatementPreprocessor.scala b/src/main/scala/millfork/compiler/AbstractStatementPreprocessor.scala
index 4373b6ad..523454eb 100644
--- a/src/main/scala/millfork/compiler/AbstractStatementPreprocessor.scala
+++ b/src/main/scala/millfork/compiler/AbstractStatementPreprocessor.scala
@@ -61,11 +61,8 @@ abstract class AbstractStatementPreprocessor(ctx: CompilationContext, statements
     val pos = stmt.position
     // generic warnings:
     stmt match {
-      case ExpressionStatement(expr@FunctionCallExpression("strzlen" | "putstrz", List(TextLiteralExpression(ch)))) =>
-        ch.last match {
-          case LiteralExpression(0, _) => //ok
-          case _ => ctx.log.warn("Passing a non-null-terminated string to a function that expects a null-terminated string.", stmt.position)
-        }
+      case ExpressionStatement(expr@FunctionCallExpression("strzlen" | "putstrz" | "strzcmp" | "strzcopy", params)) =>
+        for (param <- params) checkIfNullTerminated(stmt, param)
       case ExpressionStatement(VariableExpression(v)) =>
         val volatile = ctx.env.maybeGet[ThingInMemory](v).fold(false)(_.isVolatile)
         if (!volatile) ctx.log.warn("Pointless expression.", stmt.position)
@@ -133,6 +130,17 @@ abstract class AbstractStatementPreprocessor(ctx: CompilationContext, statements
     }
   }
 
+  private def checkIfNullTerminated(stmt: ExecutableStatement, param: Expression): Unit = {
+    param match {
+      case TextLiteralExpression(ch) =>
+        ch.last match {
+          case LiteralExpression(0, _) => //ok
+          case _ => ctx.log.warn("Passing a non-null-terminated string to a function that expects a null-terminated string.", stmt.position)
+        }
+      case _ =>
+    }
+  }
+
   def search(expr: Expression, cv: VV): VV = {
     expr match {
       case FunctionCallExpression(op, List(VariableExpression(v), arg)) if op.endsWith("=") && op != "<=" && op != ">=" =>
diff --git a/src/main/scala/millfork/compiler/mos/BuiltIns.scala b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
index cbad7ad7..72f85263 100644
--- a/src/main/scala/millfork/compiler/mos/BuiltIns.scala
+++ b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
@@ -613,7 +613,7 @@ object BuiltIns {
           AssemblyLine.variable(ctx, CMP, rva, 0))
       case _ =>
         // TODO comparing expressions
-        ctx.log.error("Too complex expressions in comparison", lhs.position)
+        ctx.log.error("Too complex expressions in comparison", lhs.position.orElse(rhs.position))
         (Nil, Nil, Nil, Nil)
     }
     val lType = MosExpressionCompiler.getExpressionType(ctx, lhs)
diff --git a/src/main/scala/millfork/env/Environment.scala b/src/main/scala/millfork/env/Environment.scala
index e2b23148..dc6ad32e 100644
--- a/src/main/scala/millfork/env/Environment.scala
+++ b/src/main/scala/millfork/env/Environment.scala
@@ -798,7 +798,26 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
           case e: ExecutableStatement => Some(e)
           case _ => None
         }
-        val needsExtraRTS = !stmt.isMacro && !stmt.assembly && (statements.isEmpty || !statements.last.isInstanceOf[ReturnStatement])
+        val paramForAutomaticReturn: List[Option[Expression]] = if (stmt.isMacro || stmt.assembly) {
+          Nil
+        } else if (statements.isEmpty) {
+          List(None)
+        } else {
+          statements.last match {
+            case _: ReturnStatement => Nil
+            case WhileStatement(VariableExpression(tr), _, _, _) =>
+              if (resultType.size > 0 && env.getBooleanConstant(tr).contains(true)) {
+                List(Some(LiteralExpression(0, 1))) // TODO: what if the loop is breakable?
+              } else List(None)
+            case DoWhileStatement(_, _, VariableExpression(tr), _) =>
+              if (resultType.size > 0 && env.getBooleanConstant(tr).contains(true)) {
+                List(Some(LiteralExpression(0, 1))) // TODO: what if the loop is breakable?
+              } else List(None)
+            case _ =>
+              // None so the compiler warns
+              List(None)
+          }
+        }
         if (stmt.isMacro) {
           if (stmt.bank.isDefined) {
             log.error("Macro functions cannot be in a defined segment", stmt.position)
@@ -823,7 +842,7 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
             env,
             stackVariablesSize,
             stmt.address.map(a => this.eval(a).getOrElse(errorConstant(s"Address of `${stmt.name}` is not a constant"))),
-            executableStatements ++ (if (needsExtraRTS) List(ReturnStatement(None).pos(executableStatements.lastOption.fold(stmt.position)(_.position))) else Nil),
+            executableStatements ++ paramForAutomaticReturn.map(param => ReturnStatement(param).pos(executableStatements.lastOption.fold(stmt.position)(_.position))),
             interrupt = stmt.interrupt,
             kernalInterrupt = stmt.kernalInterrupt,
             reentrant = stmt.reentrant,