diff --git a/core/src/main/scala/sbt/librarymanagement/VersionNumber.scala b/core/src/main/scala/sbt/librarymanagement/VersionNumber.scala
index aa13161d1..bee13b8f9 100644
--- a/core/src/main/scala/sbt/librarymanagement/VersionNumber.scala
+++ b/core/src/main/scala/sbt/librarymanagement/VersionNumber.scala
@@ -129,6 +129,9 @@ object VersionNumber {
      * Rule 2 we enforce with custom extractors.
      * Rule 4 we enforce by matching x = 0 & fully equals checking the two versions
      * Rule 6, 7 & 8 means version compatibility is determined by comparing the two X values
+     * Rule 9..
+     *   Dale thinks means pre-release versions are fully equals checked..
+     *   Eugene thinks means pre-releases before 1.0.0 are not compatible, if not they are..
      */
     def isCompatible(v1: VersionNumber, v2: VersionNumber): Boolean =
       doIsCompat(dropBuildMetadata(v1), dropBuildMetadata(v2))
@@ -136,6 +139,7 @@ object VersionNumber {
     private[this] def doIsCompat(v1: VersionNumber, v2: VersionNumber): Boolean =
       (v1, v2) match {
         case (NormalVersion(0, _, _), NormalVersion(0, _, _))   => v1 == v2 // R4
+        case (NormalVersion(_, 0, 0), NormalVersion(_, 0, 0))   => v1 == v2 // R9 maybe?
         case (NormalVersion(x1, _, _), NormalVersion(x2, _, _)) => x1 == x2 // R6, R7 & R8
         case _                                                  => false
       }
@@ -168,6 +172,7 @@ object VersionNumber {
 
     private[this] def doIsCompat(v1: VersionNumber, v2: VersionNumber): Boolean = {
       (v1, v2) match {
+        case (NormalVersion(_, _, 0), NormalVersion(_, _, 0))     => v1 == v2 // R9 maybe?
         case (NormalVersion(x1, y1, _), NormalVersion(x2, y2, _)) => (x1 == x2) && (y1 == y2)
         case _                                                    => false
       }
diff --git a/core/src/test/scala/sbt/librarymanagement/VersionNumberSpec.scala b/core/src/test/scala/sbt/librarymanagement/VersionNumberSpec.scala
index 6401f386d..eab52f6c9 100644
--- a/core/src/test/scala/sbt/librarymanagement/VersionNumberSpec.scala
+++ b/core/src/test/scala/sbt/librarymanagement/VersionNumberSpec.scala
@@ -26,12 +26,12 @@ class VersionNumberSpec extends FreeSpec with Matchers with Inside {
     assertIsCompatibleWith(v, "1.0.1", SemVer)
     assertIsCompatibleWith(v, "1.1.1", SemVer)
     assertIsNotCompatibleWith(v, "2.0.0", SemVer)
-    assertIsCompatibleWith(v, "1.0.0-M1", SemVer)
+    assertIsNotCompatibleWith(v, "1.0.0-M1", SemVer)
 
     assertIsCompatibleWith(v, "1.0.1", SecondSegment)
     assertIsNotCompatibleWith(v, "1.1.1", SecondSegment)
     assertIsNotCompatibleWith(v, "2.0.0", SecondSegment)
-    assertIsCompatibleWith(v, "1.0.0-M1", SecondSegment)
+    assertIsNotCompatibleWith(v, "1.0.0-M1", SecondSegment)
   }
 
   version("1.0.0.0") { v =>
@@ -49,7 +49,7 @@ class VersionNumberSpec extends FreeSpec with Matchers with Inside {
     assertIsNotCompatibleWith(v, "0.12.1", SemVer)
     assertIsNotCompatibleWith(v, "0.12.1-M1", SemVer)
 
-    assertIsCompatibleWith(v, "0.12.0-RC1", SecondSegment)
+    assertIsNotCompatibleWith(v, "0.12.0-RC1", SecondSegment)
     assertIsCompatibleWith(v, "0.12.1", SecondSegment)
     assertIsCompatibleWith(v, "0.12.1-M1", SecondSegment)
   }
@@ -63,7 +63,7 @@ class VersionNumberSpec extends FreeSpec with Matchers with Inside {
     assertIsCompatibleWith(v, "0.1.0-SNAPSHOT+001", SemVer)
 
     assertIsCompatibleWith(v, "0.1.0-SNAPSHOT", SecondSegment)
-    assertIsCompatibleWith(v, "0.1.0", SecondSegment)
+    assertIsNotCompatibleWith(v, "0.1.0", SecondSegment)
     assertIsCompatibleWith(v, "0.1.0-SNAPSHOT+001", SecondSegment)
   }