diff --git a/Cargo.lock b/Cargo.lock
index aed6982d..bb18db92 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1979,6 +1979,15 @@ dependencies = [
  "rawpointer",
 ]
 
+[[package]]
+name = "no-std-compat"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b93853da6d84c2e3c7d730d6473e8817692dd89be387eb01b94d7f108ecb5b8c"
+dependencies = [
+ "spin 0.5.2",
+]
+
 [[package]]
 name = "nodrop"
 version = "0.1.14"
@@ -2200,6 +2209,9 @@ name = "once_cell"
 version = "1.20.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1261fe7e33c73b354eab43b1273a57c8f967d0391e80353e51f764ac02cf6775"
+dependencies = [
+ "portable-atomic",
+]
 
 [[package]]
 name = "oorandom"
@@ -3528,13 +3540,14 @@ dependencies = [
 
 [[package]]
 name = "rhai"
-version = "1.19.0"
+version = "1.20.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "61797318be89b1a268a018a92a7657096d83f3ecb31418b9e9c16dcbb043b702"
+checksum = "8867cfc57aaf2320b60ec0f4d55603ac950ce852e6ab6b9109aa3d626a4dd7ea"
 dependencies = [
  "ahash",
  "bitflags 2.6.0",
  "instant",
+ "no-std-compat",
  "num-traits 0.2.19",
  "once_cell",
  "rhai_codegen",
@@ -3564,7 +3577,7 @@ dependencies = [
  "cfg-if",
  "getrandom",
  "libc",
- "spin",
+ "spin 0.9.8",
  "untrusted",
  "windows-sys 0.52.0",
 ]
@@ -4047,6 +4060,12 @@ dependencies = [
  "smallvec",
 ]
 
+[[package]]
+name = "spin"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e63cff320ae2c57904679ba7cb63280a3dc4613885beafb148ee7bf9aa9042d"
+
 [[package]]
 name = "spin"
 version = "0.9.8"
diff --git a/pywr-core/Cargo.toml b/pywr-core/Cargo.toml
index e256035a..6eed966e 100644
--- a/pywr-core/Cargo.toml
+++ b/pywr-core/Cargo.toml
@@ -31,7 +31,7 @@ chrono = { workspace = true }
 polars = { workspace = true }
 pyo3 = { workspace = true, features = ["chrono", "macros"] }
 rayon = "1.6"
-rhai = { version = "1.19", features = ["sync"] }
+rhai = { version = "1.20", features = ["sync"] }
 ocl = { version = "0.19", optional = true }
 rand = { workspace = true }
 rand_distr = "0.4"