diff --git a/compiler/src/dotty/tools/dotc/Run.scala b/compiler/src/dotty/tools/dotc/Run.scala
index acd885dd3afe..e9ec9dd3d96b 100644
--- a/compiler/src/dotty/tools/dotc/Run.scala
+++ b/compiler/src/dotty/tools/dotc/Run.scala
@@ -24,7 +24,6 @@ import scala.util.control.NonFatal
 /** A compiler run. Exports various methods to compile source files */
 class Run(comp: Compiler)(implicit ctx: Context) {
 
-  assert(comp.phases.last.last.id <= Periods.MaxPossiblePhaseId)
   assert(ctx.runId <= Periods.MaxPossibleRunId)
 
   var units: List[CompilationUnit] = _
diff --git a/compiler/src/dotty/tools/dotc/core/Phases.scala b/compiler/src/dotty/tools/dotc/core/Phases.scala
index 16f281a42ee9..409827f39ad6 100644
--- a/compiler/src/dotty/tools/dotc/core/Phases.scala
+++ b/compiler/src/dotty/tools/dotc/core/Phases.scala
@@ -348,6 +348,7 @@ object Phases {
     protected[Phases] def init(base: ContextBase, start: Int, end:Int): Unit = {
       if (start >= FirstPhaseId)
         assert(myPeriod == Periods.InvalidPeriod, s"phase $this has already been used once; cannot be reused")
+      assert(start <= Periods.MaxPossiblePhaseId, s"Too many phases, Period bits overflow")
       myBase = base
       myPeriod = Period(NoRunId, start, end)
       myErasedTypes  = prev.getClass == classOf[Erasure]      || prev.erasedTypes