diff --git a/src/internalHelpers/errors.md b/src/internalHelpers/errors.md
index 86137541..08f6f104 100644
--- a/src/internalHelpers/errors.md
+++ b/src/internalHelpers/errors.md
@@ -302,3 +302,7 @@ CSS variable not found and no default was provided.
 ## 75
 
 important requires a valid style object, got a %s instead.
+
+## 76
+
+fromSize and toSize must be provided as stringified numbers with the same units as minScreen and maxScreen.
diff --git a/src/mixins/between.js b/src/mixins/between.js
index c3597e50..abbf27c7 100644
--- a/src/mixins/between.js
+++ b/src/mixins/between.js
@@ -54,6 +54,10 @@ export default function between(
     throw new PolishedError(48)
   }
 
+  if (fromSizeUnit !== minScreenUnit || toSizeUnit !== maxScreenUnit) {
+    throw new PolishedError(76)
+  }
+
   const slope = (unitlessFromSize - unitlessToSize)
     / (unitlessMinScreen - unitlessMaxScreen)
   const base = unitlessToSize - slope * unitlessMaxScreen
diff --git a/src/mixins/test/__snapshots__/between.test.js.snap b/src/mixins/test/__snapshots__/between.test.js.snap
index 6752db14..b7574a31 100644
--- a/src/mixins/test/__snapshots__/between.test.js.snap
+++ b/src/mixins/test/__snapshots__/between.test.js.snap
@@ -3,7 +3,3 @@
 exports[`between should return a valid calc formula when not passed min/max screen sizes 1`] = `"calc(-9.09px + 9.09vw)"`;
 
 exports[`between should return a valid calc formula when passed min/max screen sizes 1`] = `"calc(-33.33px + 13.33vw)"`;
-
-exports[`between should return a valid calc formula when passed unitless to/from values as numbers 1`] = `"calc(-9.09 + 9.09vw)"`;
-
-exports[`between should return a valid calc formula when passed unitless to/from values as strings 1`] = `"calc(-9.09 + 9.09vw)"`;
diff --git a/src/mixins/test/between.test.js b/src/mixins/test/between.test.js
index 6458314b..ee532034 100644
--- a/src/mixins/test/between.test.js
+++ b/src/mixins/test/between.test.js
@@ -10,13 +10,6 @@ describe('between', () => {
     expect(between('20px', '100px')).toMatchSnapshot()
   })
 
-  it('should return a valid calc formula when passed unitless to/from values as numbers', () => {
-    expect(between(20, 100)).toMatchSnapshot()
-  })
-
-  it('should return a valid calc formula when passed unitless to/from values as strings', () => {
-    expect(between('20', '100')).toMatchSnapshot()
-  })
   // Errors
   it('should throw an error when not passed min/max screen size as a string', () => {
     expect(() => {
@@ -44,4 +37,22 @@ describe('between', () => {
       'fromSize and toSize must be provided as stringified numbers with the same units.',
     )
   })
+
+  it('should throw an error when passed to/from size with different units than mix/max screen', () => {
+    expect(() => {
+      // $FlowFixMe
+      between('1em', '100em', '400px', '1000px')
+    }).toThrow(
+      'fromSize and toSize must be provided as stringified numbers with the same units as minScreen and maxScreen.',
+    )
+  })
+
+  it('should throw an error when passed to/from size with no units but mix/max with units', () => {
+    expect(() => {
+      // $FlowFixMe
+      between(20, 100)
+    }).toThrow(
+      'fromSize and toSize must be provided as stringified numbers with the same units as minScreen and maxScreen.',
+    )
+  })
 })