diff --git a/zstd/src/main/java/io/github/dfa1/zstd/ZstdDecompressCtx.java b/zstd/src/main/java/io/github/dfa1/zstd/ZstdDecompressCtx.java
index bfa42d1..f864695 100644
--- a/zstd/src/main/java/io/github/dfa1/zstd/ZstdDecompressCtx.java
+++ b/zstd/src/main/java/io/github/dfa1/zstd/ZstdDecompressCtx.java
@@ -10,6 +10,8 @@
 /// state allocation. Not thread-safe: confine an instance to one thread or pool it.
 public final class ZstdDecompressCtx extends NativeObject {
 
+    private static final String COMPRESSED = "compressed";
+
     /// Creates a new decompression context.
     public ZstdDecompressCtx() {
         super(create());
@@ -53,7 +55,7 @@ public ZstdDecompressCtx windowLogMax(int windowLogMax) {
     /// @param maxSize    upper bound on the decompressed length
     /// @return the original bytes
     public byte[] decompress(byte[] compressed, int maxSize) {
-        Objects.requireNonNull(compressed, "compressed");
+        Objects.requireNonNull(compressed, COMPRESSED);
         try (Arena arena = Arena.ofConfined()) {
             MemorySegment in = Zstd.copyIn(arena, compressed);
             MemorySegment out = arena.allocate(Math.max(maxSize, 1));
@@ -74,7 +76,7 @@ public byte[] decompress(byte[] compressed, int maxSize) {
     /// @param dict       the dictionary the frame was compressed against
     /// @return the original bytes
     public byte[] decompress(byte[] compressed, int maxSize, ZstdDictionary dict) {
-        Objects.requireNonNull(compressed, "compressed");
+        Objects.requireNonNull(compressed, COMPRESSED);
         Objects.requireNonNull(dict, "dict");
         try (Arena arena = Arena.ofConfined()) {
             MemorySegment in = Zstd.copyIn(arena, compressed);
@@ -94,7 +96,7 @@ public byte[] decompress(byte[] compressed, int maxSize, ZstdDictionary dict) {
     /// @param dict       the pre-digested decompression dictionary
     /// @return the original bytes
     public byte[] decompress(byte[] compressed, int maxSize, ZstdDecompressDict dict) {
-        Objects.requireNonNull(compressed, "compressed");
+        Objects.requireNonNull(compressed, COMPRESSED);
         Objects.requireNonNull(dict, "dict");
         try (Arena arena = Arena.ofConfined()) {
             MemorySegment in = Zstd.copyIn(arena, compressed);
diff --git a/zstd/src/main/java/io/github/dfa1/zstd/ZstdDictionary.java b/zstd/src/main/java/io/github/dfa1/zstd/ZstdDictionary.java
index 1eae3c5..42cbc72 100644
--- a/zstd/src/main/java/io/github/dfa1/zstd/ZstdDictionary.java
+++ b/zstd/src/main/java/io/github/dfa1/zstd/ZstdDictionary.java
@@ -34,6 +34,7 @@ public final class ZstdDictionary {
 
     private static final String FIELD_NB_THREADS = "nbThreads";
     private static final String FIELD_COMPRESSION_LEVEL = "compressionLevel";
+    private static final String SAMPLES = "samples";
 
     // ZDICT_cover_params_t fields: unsigned k, d, steps, nbThreads; double
     // splitPoint; unsigned shrinkDict, shrinkDictMaxRegression; then a nested
@@ -94,7 +95,7 @@ public static ZstdDictionary of(byte[] raw) {
     /// @return the trained dictionary
     /// @throws ZstdException if training fails (commonly: not enough sample data)
     public static ZstdDictionary train(List<byte[]> samples, int maxDictBytes) {
-        Objects.requireNonNull(samples, "samples");
+        Objects.requireNonNull(samples, SAMPLES);
         if (samples.isEmpty()) {
             throw new ZstdException("cannot train a dictionary from zero samples");
         }
@@ -178,7 +179,7 @@ public static ZstdDictionary trainFastCover(List<byte[]> samples, int maxDictByt
 
     private static ZstdDictionary optimize(List<byte[]> samples, int maxDictBytes,
                                            int compressionLevel, boolean fast) {
-        Objects.requireNonNull(samples, "samples");
+        Objects.requireNonNull(samples, SAMPLES);
         if (samples.isEmpty()) {
             throw new ZstdException("cannot train a dictionary from zero samples");
         }
@@ -233,7 +234,7 @@ private static ZstdDictionary optimize(List<byte[]> samples, int maxDictBytes,
     public static ZstdDictionary finalizeFrom(byte[] content, List<byte[]> samples,
                                               int maxDictBytes, int compressionLevel) {
         Objects.requireNonNull(content, "content");
-        Objects.requireNonNull(samples, "samples");
+        Objects.requireNonNull(samples, SAMPLES);
         if (samples.isEmpty()) {
             throw new ZstdException("cannot finalise a dictionary from zero samples");
         }