diff --git a/packages/mobx/src/core/derivation.ts b/packages/mobx/src/core/derivation.ts
index 66e535312..8eea4afa3 100644
--- a/packages/mobx/src/core/derivation.ts
+++ b/packages/mobx/src/core/derivation.ts
@@ -167,9 +167,12 @@ export function checkIfStateReadsAreAllowed(observable: IObservable) {
 export function trackDerivedFunction<T>(derivation: IDerivation, f: () => T, context: any) {
     const prevAllowStateReads = allowStateReadsStart(true)
     changeDependenciesStateTo0(derivation)
-    // pre allocate array allocation + 20% extra room for variation in deps
-    // array will be trimmed by bindDependencies
-    derivation.newObserving_ = new Array(Math.ceil(derivation.observing_.length * 1.2))
+    // Preallocate array; will be trimmed by bindDependencies.
+    derivation.newObserving_ = new Array(
+        derivation.observing_.length
+            ? Math.ceil(derivation.observing_.length * 1.2) // 20% extra room for variation in deps
+            : 100 // Establishing initial dependencies, preallocate constant space.
+    )
     derivation.unboundDepsCount_ = 0
     derivation.runId_ = ++globalState.runId
     const prevTracking = globalState.trackingDerivation