diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## [v1.5.1.1](https://github.com/diagrams/diagrams-core/tree/v1.5.1.1) (2023-11-15)
+
+* Allow `base-4.19` and test on GHC 9.8
+* Fix more warnings
+
 ## [v1.5.1](https://github.com/diagrams/diagrams-core/tree/v1.5.1) (2023-05-11)
 
 * Allow `base-4.18` and test on GHC 9.6 (thanks to @sergv)
diff --git a/README.markdown b/README.markdown
--- a/README.markdown
+++ b/README.markdown
@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/diagrams/diagrams-core.png?branch=master)](http://travis-ci.org/diagrams/diagrams-core)
+[![Build Status](https://github.com/diagrams/diagrams-core/actions/workflows/haskell-ci.yml/badge.svg)](https://github.com/diagrams/diagrams-core/actions/workflows/haskell-ci.yml)
 
 The core modules defining the basic data structures and algorithms for
 [diagrams](http://projects.haskell.org/diagrams), a Haskell embedded
diff --git a/diagrams-core.cabal b/diagrams-core.cabal
--- a/diagrams-core.cabal
+++ b/diagrams-core.cabal
@@ -1,5 +1,5 @@
 Name:                diagrams-core
-Version:             1.5.1
+Version:             1.5.1.1
 Synopsis:            Core libraries for diagrams EDSL
 Description:         The core modules underlying diagrams,
                      an embedded domain-specific language
@@ -15,7 +15,7 @@
 Cabal-version:       1.18
 Extra-source-files:  diagrams/*.svg
 extra-doc-files:     diagrams/*.svg, CHANGELOG.md, README.markdown
-Tested-with:         GHC ==8.4.4 || ==8.6.5 || ==8.8.4 || ==8.10.7 || ==9.0.2 || ==9.2.4 || ==9.4.1
+Tested-with:         GHC ==8.4.4 || ==8.6.5 || ==8.8.4 || ==8.10.7 || ==9.0.2 || ==9.2.8 || ==9.4.7 || ==9.6.3 || ==9.8.1
 Source-repository head
   type:     git
   location: git://github.com/diagrams/diagrams-core.git
@@ -36,7 +36,7 @@
                        Diagrams.Core.Types,
                        Diagrams.Core.V
 
-  Build-depends:       base >= 4.11 && < 4.19,
+  Build-depends:       base >= 4.11 && < 4.20,
                        containers >= 0.4.2 && < 0.7,
                        unordered-containers >= 0.2 && < 0.3,
                        semigroups >= 0.8.4 && < 0.21,
diff --git a/src/Diagrams/Core/Measure.hs b/src/Diagrams/Core/Measure.hs
--- a/src/Diagrams/Core/Measure.hs
+++ b/src/Diagrams/Core/Measure.hs
@@ -126,7 +126,6 @@
 
 instance Monoid a => Monoid (Measured n a) where
   mempty  = pure mempty
-  mappend = liftA2 mappend
 
 instance Distributive (Measured n) where
   distribute a = Measured $ \x -> fmap (\(Measured m) -> m x) a
diff --git a/src/Diagrams/Core/Transform.hs b/src/Diagrams/Core/Transform.hs
--- a/src/Diagrams/Core/Transform.hs
+++ b/src/Diagrams/Core/Transform.hs
@@ -6,10 +6,9 @@
 {-# LANGUAGE ScopedTypeVariables        #-}
 {-# LANGUAGE TypeFamilies               #-}
 {-# LANGUAGE TypeOperators              #-}
-{-# LANGUAGE TypeSynonymInstances       #-}
 {-# LANGUAGE UndecidableInstances       #-}
-{-# OPTIONS_GHC -fno-warn-unused-imports       #-}
------------------------------------------------------------------------------
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
+
 -- |
 -- Module      :  Diagrams.Core.Transform
 -- Copyright   :  (c) 2011-2015 diagrams-core team (see LICENSE)
@@ -232,21 +231,20 @@
 
 -- Remove the nth element from a list
 remove :: Int -> [a] -> [a]
-remove n xs = ys ++ tail zs
+remove n xs = ys ++ zs
   where
-    (ys, zs) = splitAt n xs
+    (ys, _ : zs) = splitAt n xs
 
--- Minor matrix of cofactore C(i,j)
+-- Minor matrix of cofactor C(i,j)
 minor :: Int -> Int -> [[a]] -> [[a]]
 minor i j xs = remove j $ map (remove i) xs
 
 -- The determinant of a square matrix represented as a list of lists
 -- representing column vectors, that is [column].
 det :: Num a => [[a]] -> a
-det (a:[]) = head a
-det m = sum [(-1)^i * (c1 !! i) * det (minor i 0 m) | i <- [0 .. (n-1)]]
+det [a : _] = a
+det m@(c1 : _) = sum [(-1) ^ i * (c1 !! i) * det (minor i 0 m) | i <- [0 .. (n - 1)]]
   where
-    c1 = head m
     n = length m
 
 -- | Convert a vector v to a list of scalars.