diff --git a/.eslintrc.yaml b/.eslintrc.yaml
index 51926d896..93a1c56b3 100644
--- a/.eslintrc.yaml
+++ b/.eslintrc.yaml
@@ -22,8 +22,12 @@ globals:
 rules:
   # Code quality rules
   eqeqeq: error
+  "@typescript-eslint/consistent-type-assertions": ["error", { assertionStyle: 'never' }]
   "@typescript-eslint/no-empty-function": ["error", { "allow": ["arrowFunctions"] }]
   "@typescript-eslint/no-explicit-any": off # Allow explicit any to make incremental TypeScript adoption easier.
+  # TODO: use strict ruleset which includes no-non-null-assertion.
+  # Not possible while strictNullChecks is disabled in tsconfig.
+  "@typescript-eslint/no-non-null-assertion": "error"
   no-unused-vars: off
   "@typescript-eslint/no-unused-vars": ["error", { "argsIgnorePattern": "^_", "destructuredArrayIgnorePattern": "^_" }]
   no-use-before-define: off
diff --git a/src/util/entropyCreateStateFromJsons.ts b/src/util/entropyCreateStateFromJsons.ts
index bd042373e..0745c532a 100644
--- a/src/util/entropyCreateStateFromJsons.ts
+++ b/src/util/entropyCreateStateFromJsons.ts
@@ -103,7 +103,7 @@ export const genomeMap = (annotations: JsonAnnotations): GenomeAnnotation => {
     .map(([annotationKey, annotation]) => {
       const geneName = annotation.gene || annotationKey;
       if (!(geneName in annotationsPerGene)) annotationsPerGene[geneName] = {};
-      const gene = annotationsPerGene[geneName] as JsonAnnotations;  // TODO - why do I need to cast?
+      const gene = annotationsPerGene[geneName];
       gene[annotationKey] = annotation;
     })
 
@@ -167,7 +167,7 @@ function validColor(color:(string|undefined)) {
 function* nextColorGenerator() {
   let i=0;
   while (true) {
-    yield genotypeColors[i++] as string;
+    yield genotypeColors[i++];
     if (i===genotypeColors.length) i=0;
   }
 }