diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index b03ace7f8d50..b74ad17a1311 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -89,7 +89,6 @@ object ErrorReporting {
       if (tree.tpe.widen.exists)
         i"${exprStr(tree)} does not take ${kind}parameters"
       else {
-        if (ctx.settings.Ydebug.value) new FatalError("").printStackTrace()
         i"undefined: $tree # ${tree.uniqueId}: ${tree.tpe.toString} at ${ctx.phase}"
       }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index f66f5e996b90..544c8b49ec98 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -464,7 +464,8 @@ object Nullables with
    */
   def postProcessByNameArgs(fn: TermRef, app: Tree)(given ctx: Context): Tree =
     fn.widen match
-      case mt: MethodType if mt.paramInfos.exists(_.isInstanceOf[ExprType]) =>
+      case mt: MethodType
+      if mt.paramInfos.exists(_.isInstanceOf[ExprType]) && !fn.symbol.is(Inline) =>
         app match
           case Apply(fn, args) =>
             val dropNotNull = new TreeMap with
@@ -487,10 +488,10 @@ object Nullables with
                 case _ => super.typedUnadapted(t, pt, locked)
 
             def postProcess(formal: Type, arg: Tree): Tree =
-              val arg1 = dropNotNull.transform(arg)
+              val nestedCtx = ctx.fresh.setNewTyperState()
+              val arg1 = dropNotNull.transform(arg)(given nestedCtx)
               if arg1 eq arg then arg
               else
-                val nestedCtx = ctx.fresh.setNewTyperState()
                 val arg2 = retyper.typed(arg1, formal)(given nestedCtx)
                 if nestedCtx.reporter.hasErrors || !(arg2.tpe <:< formal) then
                   ctx.error(em"""This argument was typed using flow assumptions about mutable variables
@@ -506,7 +507,10 @@ object Nullables with
 
             def recur(formals: List[Type], args: List[Tree]): List[Tree] = (formals, args) match
               case (formal :: formalsRest, arg :: argsRest) =>
-                val arg1 = postProcess(formal.widenExpr.repeatedToSingle, arg)
+                val arg1 =
+                  if formal.isInstanceOf[ExprType]
+                  then postProcess(formal.widenExpr.repeatedToSingle, arg)
+                  else arg
                 val argsRest1 = recur(
                   if formal.isRepeatedParam then formals else formalsRest,
                   argsRest)
diff --git a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
index 09cf453e483e..d31e2b6c1bfd 100644
--- a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
+++ b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
@@ -411,6 +411,7 @@ trait TypeAssigner {
         else
           errorType(i"wrong number of arguments at ${ctx.phase.prev} for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.sourcePos)
       case t =>
+        if (ctx.settings.Ydebug.value) new FatalError("").printStackTrace()
         errorType(err.takesNoParamsStr(fn, ""), tree.sourcePos)
     }
     ConstFold(tree.withType(ownType))
diff --git a/tests/neg-custom-args/explicit-nulls/byname-nullables1.check b/tests/neg-custom-args/explicit-nulls/byname-nullables1.check
new file mode 100644
index 000000000000..e4a46bf8322f
--- /dev/null
+++ b/tests/neg-custom-args/explicit-nulls/byname-nullables1.check
@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/explicit-nulls/byname-nullables1.scala:10:6 --------------------------------------------
+10 |    f(x.fld != null) // error
+   |      ^^^^^^^^^^^^^
+   |      This argument was typed using flow assumptions about mutable variables
+   |      but it is passed to a by-name parameter where such flow assumptions are unsound.
+   |      Wrapping the argument in `byName(...)` fixes the problem by disabling the flow assumptions.
+   |
+   |      `byName` needs to be imported from the `scala.compiletime` package.
diff --git a/tests/neg-custom-args/explicit-nulls/byname-nullables1.scala b/tests/neg-custom-args/explicit-nulls/byname-nullables1.scala
new file mode 100644
index 000000000000..eb28a5af96c0
--- /dev/null
+++ b/tests/neg-custom-args/explicit-nulls/byname-nullables1.scala
@@ -0,0 +1,11 @@
+def f(op: => Boolean): Unit = ()
+def f(op: Int): Unit = ()
+
+class C with
+  var fld: String | Null = null
+
+def test() =
+  var x: C | Null = C()
+  if x != null then
+    f(x.fld != null) // error
+    require(x.fld != null, "error")    // ok
\ No newline at end of file