R32_FLOAT texture not sampled with texelFetch

I am wanting to use a texture to pass some data to my vertex shader. As a proof of concept, I am constructing a simple 64x64 GL_R32F texture where all pixels have the exact as value as follows:

Urho3D::PODVector<float> heightData( 64u * 64u, 50.f );

Urho3D::Texture *texture = new Urho3D::Texture( context_ );
texture->SetFilterMode( Urho3D::FILTER_NEAREST );
texture->SetAddressMode( Urho3D::COORD_U, Urho3D::ADDRESS_CLAMP );
texture->SetAddressMode( Urho3D::COORD_V, Urho3D::ADDRESS_CLAMP );
texture->SetSize( 64u, 64u, Urho3D::Graphics::GetFloat32Format() );
texture->SetData( 0u, 0u, 0u, 64u, 64u, heightData.Buffer() );

I then bind this texture to my material:

material->SetTexture( Urho3D::TU_SPECULAR, texture );

Inside my Vertex Shader:

// all the normal includes

varying vec2 vTexCoord;
varying vec4 vWorldPos;
varying float vHeight; // added here to see value in renderdoc

uniform sampler2D sSpecMap;

void VS()
  // Simply pull the r-channel value from the 0,0 index in the float array
  float height = texelFetch( sSpecMap, ivec2( 0, 0 ), 0 ).r;
  vHeight = height;

  mat4 modelMatrix = iModelMatrix;
  vec3 worldPos = GetWorldPos( modelMatrix );
  worldPos.y = height;

  gl_Position = GetClipPos( worldPos );
  vTexCoord = GetTexCoord( iTexCoord );
  vWorldPos = vec4( worldPos, GetDepth( gl_Position ) );

When I double check the output from the VS inside RenderDoc, I find that texelFetch always returns 0. I’ve even checked the actual full vec4 returned from texelFetch and its always 0,0,0,1.

Now if I define uniform sampler2D sDiffMap inside the COMPILEVS macro and then swap using the sDiffMap uniform rather than the sSpecMap uniform then i get non-zero values, but these are the expected [0…1] range values for my base diffuse layer texture.

I’ve checked the float array texture in RenderDoc, its a 64x64 texture, R32_FLOAT format, where all pixels return a value of 50 when I query the texture.

So at this point I’m not sure what exactly I’ve done incorrectly. On all accounts it would appear that I have created my float-array texture correctly, its bound to the vertex shader adequately, but for whatever reason texelFetch refuses to return the expected value.

Any thoughts on what I’ve likely done incorrectly?